Quarterly report pursuant to Section 13 or 15(d)

Commitments and Contingencies (Details)

Commitments and Contingencies (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Commitments and Contingencies [Abstract]    
Royalties paid, minimum (in hundredths) 0.50%xoma_RoyaltiesPaidPercentageMinimum  
Royalties paid, maximum (in hundredths) 5.00%xoma_RoyaltiesPaidPercentageMaximum  
Sublicense fees paid, maximum (in hundredths) 40.00%xoma_RoyaltiesPaidSublicenseFeePercentageMaximum  
Estimate of milestone payments $ 77.3xoma_ContractualObligationEstimatedMilestonePayments  
Leases [Abstract]    
Total rental expense $ 0.9us-gaap_LeaseAndRentalExpense $ 0.9us-gaap_LeaseAndRentalExpense