| 
           Fair Value Measurements, Warrant Liabilities, Changes in Level 3, and Outstanding Debt (Details) (USD $)
  
         | 
        3 Months Ended | 
        12 Months Ended | 
      
      
        | 
           Mar. 31, 2015 
         | 
        
           Mar. 31, 2014 
         | 
        
           Dec. 31, 2014 
         | 
      
      
        | Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Reclassification of contingent warrant liability to equity upon exercise of warrants | 
        
$ 0xoma_ReclassificationOfContingentWarrantLiabilityToEquityExerciseOfWarrants
 | 
        
$ (2,525,000)xoma_ReclassificationOfContingentWarrantLiabilityToEquityExerciseOfWarrants
 | 
         
 | 
      
      
        | Carrying Value [Member] | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | 
         
 | 
         
 | 
         
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        | Outstanding debt | 
        
47,189,000us-gaap_DebtInstrumentFairValue / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 | 
         
 | 
        
35,308,000us-gaap_DebtInstrumentFairValue / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 | 
      
      
        | Fair Value [Member] | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Outstanding debt | 
        
47,668,000us-gaap_DebtInstrumentFairValue / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
 | 
         
 | 
        
36,461,000us-gaap_DebtInstrumentFairValue / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
 | 
      
      
        | Warrant Liabilities [Member] | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair Value Inputs, Liabilities, Quantitative Information [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Risk-free interest rate (in hundredths) | 
        
0.55%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
         
 | 
         
 | 
      
      
        | Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Beginning balance | 
        
31,828,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
        
69,869,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
        
69,869,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
      
      
        | Reclassification of contingent warrant liability to equity upon exercise of warrants | 
        
0xoma_ReclassificationOfContingentWarrantLiabilityToEquityExerciseOfWarrants / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
        
(2,525,000)xoma_ReclassificationOfContingentWarrantLiabilityToEquityExerciseOfWarrants / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
         
 | 
      
      
        | Net increase (decrease) in estimated fair value of contingent warrant liabilities upon revaluation | 
        
40,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
        
(20,002,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
         
 | 
      
      
        | Ending balance | 
        
31,868,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
        
47,342,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember
 | 
         
 | 
      
      
        | Warrant Liabilities [Member] | Minimum [Member] | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair Value Inputs, Liabilities, Quantitative Information [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Expected volatility (in hundredths) | 
        
73.90%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
 | 
         
 | 
        
69.60%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
 | 
      
      
        | Risk-free interest rate (in hundredths) | 
         
 | 
         
 | 
        
0.03%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
 | 
      
      
        | Expected term | 
        1 year 8 months 8 days
 | 
         
 | 
        0 years 1 month 2 days
 | 
      
      
        | Warrant Liabilities [Member] | Maximum [Member] | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair Value Inputs, Liabilities, Quantitative Information [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Expected volatility (in hundredths) | 
        
74.80%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
 | 
         
 | 
        
72.90%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
 | 
      
      
        | Risk-free interest rate (in hundredths) | 
         
 | 
         
 | 
        
0.67%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = xoma_ContingentWarrantLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
 | 
      
      
        | Expected term | 
        1 year 11 months 8 days
 | 
         
 | 
        2 years 2 months 8 days
 | 
      
      
        | Foreign Exchange Option [Member] | Maximum [Member] | 
         
 | 
         
 | 
         
 | 
      
      
        | Fair Value Inputs, Liabilities, Quantitative Information [Line Items] | 
         
 | 
         
 | 
         
 | 
      
      
        | Estimated fair value of the foreign exchange options | 
        
$ 100us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_RangeAxis = us-gaap_MaximumMember
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