| 
           Long-Term Debt and Other Financings (Details)
  
         | 
        3 Months Ended | 
        12 Months Ended | 
        0 Months Ended | 
        3 Months Ended | 
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         | 
        3 Months Ended | 
         | 
        1 Months Ended | 
        3 Months Ended | 
        12 Months Ended | 
         | 
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        3 Months Ended | 
        0 Months Ended | 
         | 
        0 Months Ended | 
         | 
        0 Months Ended | 
         | 
        3 Months Ended | 
         | 
        3 Months Ended | 
      
      
        | 
           Mar. 31, 2015
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          USD ($) 
         | 
        
           Dec. 31, 2014
 
          USD ($) 
         | 
        
           Sep. 27, 2012
 
          General Electric Capital Corporation Term Loan [Member] 
         | 
        
           Mar. 31, 2015
 
          General Electric Capital Corporation Term Loan [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          General Electric Capital Corporation Term Loan [Member]
 
          USD ($) 
         | 
        
           Sep. 27, 2012
 
          General Electric Capital Corporation Term Loan [Member]
 
          USD ($) 
         | 
        
           Dec. 31, 2011
 
          General Electric Capital Corporation Term Loan [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2015
 
          Hercules Loan [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Hercules Loan [Member]
 
          USD ($) 
         | 
        
           Feb. 27, 2015
 
          Hercules Loan [Member]
 
          USD ($) 
         | 
        
           Jan. 31, 2015
 
          Servier Loan [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2015
 
          Servier Loan [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Servier Loan [Member]
 
          USD ($) 
         | 
        
           Dec. 31, 2011
 
          Servier Loan [Member]
 
          USD ($) 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche 
         | 
        
           Dec. 31, 2014
 
          Servier Loan [Member]
 
          USD ($) 
         | 
        
           Dec. 31, 2011
 
          Servier Loan [Member]
 
          EUR (€) 
         | 
        
           Mar. 31, 2015
 
          Servier Loan [Member]
 
          Minimum [Member] 
         | 
        
           Mar. 31, 2015
 
          Servier Loan [Member]
 
          Maximum [Member] 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche One [Member] 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche One [Member]
 
          EUR (€) 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche Two [Member] 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche Two [Member]
 
          EUR (€) 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche Three [Member] 
         | 
        
           Jan. 09, 2015
 
          Servier Loan [Member]
 
          Tranche Three [Member]
 
          EUR (€) 
         | 
        
           Mar. 31, 2015
 
          Novartis Note [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Novartis Note [Member]
 
          USD ($) 
         | 
        
           Dec. 31, 2014
 
          Novartis Note [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2015
 
          Other [Member]
 
          USD ($) 
         | 
        
           Mar. 31, 2014
 
          Other [Member]
 
          USD ($) 
         | 
      
      
        | Debt Instrument [Line Items] | 
         
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        | Maturity date | 
         
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        Sep.  01,  2018
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        Jan. 15,  2016
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        Jan. 15,  2017
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        Jan. 15,  2018
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        Jun. 30,  2015
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        | Date of agreement | 
         
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        Dec. 30,  2010
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        | Date of agreement, after amendment | 
         
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        Aug. 12,  2013
 | 
         
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        | Research and development expenses funded through loan facility, maximum (in hundredths) | 
         
 | 
         
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 | 
        
75.00%xoma_CollaborativeArrangementResearchAndDevelopmentExpenseFundedThroughDebtInstrumentPercentageMaximum / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
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        | Maximum borrowing capacity under loan agreement | 
         
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 | 
        
€ 15,000,000xoma_DebtInstrumentMaximumBorrowingCapacity / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
$ 50,000,000xoma_DebtInstrumentMaximumBorrowingCapacity / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
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 | 
      
      
        | Variable rate basis | 
         
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 | 
        The interest rate will be calculated at a rate equal to the greater of either (i) 9.40% plus the prime rate as reported from time to time in The Wall Street Journal minus 7.25%, and (ii) 9.40%.
 | 
         
 | 
         
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 | 
        Euro Inter-Bank Offered Rate ("EURIBOR")
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        six-month LIBOR
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Basis spread on variable rate (in hundredths) | 
         
 | 
         
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 | 
         
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 | 
         
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 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Period of interest resetting | 
         
 | 
         
 | 
         
 | 
         
 | 
         
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 | 
        6 months
 | 
         
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 | 
         
 | 
         
 | 
         
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        | Interest rate at period end (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
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 | 
         
 | 
         
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 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
2.35%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Outstanding principal balance | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
10,000,000us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
        
20,000,000us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
16,300,000us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
18,200,000us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
13,400,000us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
 | 
        
13,400,000us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
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        | Proceeds from loan | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
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 | 
        
19,500,000us-gaap_ProceedsFromIssuanceOfDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
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 | 
         
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 | 
         
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 | 
         
 | 
         
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        | Initial interest rate during period (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
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 | 
        
3.22%xoma_DebtInstrumentInitialInterestRateDuringPeriod / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
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 | 
      
      
        | Interest rate reset semi-annually during period (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
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 | 
         
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 | 
        
2.16%xoma_DebtInstrumentInitialInterestRateResetSemiAnnuallyDuringPeriod / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
2.31%xoma_DebtInstrumentInitialInterestRateResetSemiAnnuallyDuringPeriod / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember / us-gaap_RangeAxis = us-gaap_MinimumMember
 | 
        
3.83%xoma_DebtInstrumentInitialInterestRateResetSemiAnnuallyDuringPeriod / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember / us-gaap_RangeAxis = us-gaap_MaximumMember
 | 
         
 | 
         
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        | Number of tranches | 
         
 | 
         
 | 
         
 | 
         
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 | 
         
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 | 
         
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 | 
         
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 | 
        
3xoma_NumberOfTranches / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
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 | 
         
 | 
         
 | 
         
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        | Principal payment amount | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
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 | 
         
 | 
         
 | 
         
 | 
         
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 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
3,000,000us-gaap_DebtInstrumentAnnualPrincipalPayment / us-gaap_DebtInstrumentAxis = xoma_TrancheOneMember / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
        
5,000,000us-gaap_DebtInstrumentAnnualPrincipalPayment / us-gaap_DebtInstrumentAxis = xoma_TrancheTwoMember / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
        
7,000,000us-gaap_DebtInstrumentAnnualPrincipalPayment / us-gaap_DebtInstrumentAxis = xoma_TrancheThreeMember / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Accrued interest | 
        
(90,000)us-gaap_DebtInstrumentIncreaseAccruedInterest
 | 
        
1,764,000us-gaap_DebtInstrumentIncreaseAccruedInterest
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
200,000us-gaap_DebtInstrumentIncreaseAccruedInterest / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Euro to US Dollar exchange rates | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
1.085us-gaap_ForeignCurrencyExchangeRateTranslation1 / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
1.216us-gaap_ForeignCurrencyExchangeRateTranslation1 / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Unamortized discount on debt | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
0us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
8,900,000us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair value of loan amount | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
8,900,000us-gaap_DebtInstrumentFairValue / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
0us-gaap_DebtInstrumentFairValue / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Unrealized foreign exchange gain (loss) | 
        
1,949,000us-gaap_ForeignCurrencyTransactionGainLossUnrealized
 | 
        
66,000us-gaap_ForeignCurrencyTransactionGainLossUnrealized
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
1,900,000us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Loan discount balance | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
1,600,000us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Amortization of debt discount | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
0us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
200,000us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
        
500,000us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Carrying value of the loan | 
        
47,189,000us-gaap_LongTermDebt
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
20,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
        
20,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
        
14,700,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
16,200,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Unrealized foreign exchange gain (loss) related to re-measurement of loan discount | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
200,000xoma_UnrealizedForeignExchangeGainsRelatedToRemeasurementOfLoanDiscount / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
        
0xoma_UnrealizedForeignExchangeGainsRelatedToRemeasurementOfLoanDiscount / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Recognition of deferred revenue | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
200,000us-gaap_RecognitionOfDeferredRevenue / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
        
500,000us-gaap_RecognitionOfDeferredRevenue / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Debt issuance costs | 
         
 | 
         
 | 
        
200,000us-gaap_DebtIssuanceCosts
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
500,000us-gaap_DebtIssuanceCosts / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
6,000us-gaap_DebtIssuanceCosts / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Aggregate number of unregistered shares of common stock called by warrants (in shares) | 
         
 | 
         
 | 
         
 | 
         
 | 
        
263,158us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
        
39,346us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
        
181,268us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Warrants exercise price (in dollars per share) | 
         
 | 
         
 | 
         
 | 
         
 | 
        
$ 1.14us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
        
$ 3.54us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
        
$ 3.31us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Immediate Term for warrants exercisable (in years) | 
         
 | 
         
 | 
         
 | 
        5 years
 | 
        5 years
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair value of warrant liability | 
         
 | 
         
 | 
         
 | 
         
 | 
        
200,000us-gaap_WarrantsNotSettleableInCashFairValueDisclosure / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
        
100,000us-gaap_WarrantsNotSettleableInCashFairValueDisclosure / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
         
 | 
         
 | 
        
500,000us-gaap_WarrantsNotSettleableInCashFairValueDisclosure / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Loss on extinguishment of debt | 
        
(429,000)us-gaap_GainsLossesOnExtinguishmentOfDebt
 | 
        
0us-gaap_GainsLossesOnExtinguishmentOfDebt
 | 
         
 | 
         
 | 
        
400,000us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Outstanding principle balance and interest | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
5,500,000xoma_DebtInstrumentPaymentPrincipalAndInterest / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Percentage bearing variable rate (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
9.40%us-gaap_LongTermDebtPercentageBearingVariableInterestRate / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Reduction of percentage of interest under loan agreement (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
7.25%xoma_ReductionOfPercentageOfInterestUnderLoanAgreement / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Period of interest | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        1 year
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Amortization date | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        Jul.  01,  2016
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Date of extension of amortization | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        Oct.  01,  2016
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Period of interest amortized | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        30 days
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Prepayment fee within twelve months of maturity (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
3.00%xoma_PrepaymentFeeWithinTwelveMonthsOfMaturity / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Prepayment fee after twelve months but before twenty four months of maturity (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
2.00%xoma_PrepaymentFeeAfterTwelveMonthsButBeforeTwentyFourMonthsOfMaturity / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Prepayment fee after twenty four months of maturity (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
1.00%xoma_PrepaymentFeeAfterTwentyFourMonthsOfMaturity / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Additional interest rate in case of default (in hundredths) | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
5.00%xoma_AdditionalInterestRateInCaseOfDefault / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Final payment fee | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
1,215,000xoma_FinalPaymentFeeForLoan / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Exercisable period of warrants | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        5 years
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Aggregate future principal and final fee payments of total interest bearing obligations - long-term [Abstract] | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Nine months ending December 31, 2015 | 
        
15,140,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Year ended 2016 | 
        
9,037,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Year ended 2017 | 
        
14,659,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Year ended 2018 | 
        
17,843,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Long-term debt including current portion | 
        
56,679,000xoma_LongTermDebtTotalFuturePrincipalAndFinalFeePayments
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Less: interest, final payment and discount | 
        
(9,490,000)xoma_LongTermDebtInterestFinalPaymentAndDiscount
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Total long-term | 
        
47,189,000us-gaap_LongTermDebt
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
20,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
        
20,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
        
14,700,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
        
16,200,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Less current portion | 
        
(15,605,000)xoma_LongTermDebtIncludingFinalFeeCurrentMaturities
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Total long-term debt | 
        
31,584,000xoma_LongTermDebtIncludingFinalFeeExcludingCurrentMaturities
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Interest expense and amortization of debt issuance costs [Abstract] | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
      
      
        | Interest expense | 
        
$ 1,115,000us-gaap_InterestExpenseDebt
 | 
        
$ 1,125,000us-gaap_InterestExpenseDebt
 | 
         
 | 
         
 | 
        
$ 548,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
        
$ 448,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = us-gaap_MediumTermNotesMember
 | 
         
 | 
         
 | 
        
$ 234,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
        
$ 0us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_HerculesLoanMember
 | 
         
 | 
         
 | 
        
$ 255,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
        
$ 587,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_ServierLoanMember
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
         
 | 
        
$ 78,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
        
$ 77,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_NovartisNoteMember
 | 
         
 | 
        
$ 0us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_OtherFinancingsMember
 | 
        
$ 13,000us-gaap_InterestExpenseDebt / us-gaap_LongtermDebtTypeAxis = xoma_OtherFinancingsMember
 |