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Income Taxes (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |  |  |  |  |  |  |  |  |  |
Federal income tax (benefit) provision |  |  |  |  |  |  |  |  | $ 0 us-gaap_FederalIncomeTaxExpenseBenefitContinuingOperations | $ (14,000)us-gaap_FederalIncomeTaxExpenseBenefitContinuingOperations | $ (74,000)us-gaap_FederalIncomeTaxExpenseBenefitContinuingOperations
Total | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | 1,000 us-gaap_IncomeTaxExpenseBenefit | (15,000)us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | (14,000)us-gaap_IncomeTaxExpenseBenefit | (74,000)us-gaap_IncomeTaxExpenseBenefit
Effective Income Tax Rate Reconciliation, Percent [Abstract] |  |  |  |  |  |  |  |  |  |  |
Federal tax at statutory rate (in hundredths) |  |  |  |  |  |  |  |  | 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate | 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate | 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Warrant valuation (in hundredths) |  |  |  |  |  |  |  |  | 40.00%xoma_EffectiveIncomeTaxRateReconciliationWarrantValuationPercent | (17.00%)xoma_EffectiveIncomeTaxRateReconciliationWarrantValuationPercent | (4.00%)xoma_EffectiveIncomeTaxRateReconciliationWarrantValuationPercent
Permanent items and other (in hundredths) |  |  |  |  |  |  |  |  | (1.00%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments | 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments | (1.00%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Valuation allowance (in hundredths) |  |  |  |  |  |  |  |  | (73.00%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance | (17.00%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance | (29.00%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance
Total (in hundredths) |  |  |  |  |  |  |  |  | 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Significant components of net deferred tax assets [Abstract] |  |  |  |  |  |  |  |  |  |  |
Capitalized research and development expenses | 50,900,000 us-gaap_DeferredTaxAssetsInProcessResearchAndDevelopment |  |  |  | 49,400,000 us-gaap_DeferredTaxAssetsInProcessResearchAndDevelopment |  |  |  | 50,900,000 us-gaap_DeferredTaxAssetsInProcessResearchAndDevelopment | 49,400,000 us-gaap_DeferredTaxAssetsInProcessResearchAndDevelopment |
Net operating loss carryforwards | 105,000,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards |  |  |  | 78,400,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards |  |  |  | 105,000,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards | 78,400,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards |
Research and development and other credit carryforwards | 12,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwards |  |  |  | 8,800,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwards |  |  |  | 12,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwards | 8,800,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwards |
Other | 22,100,000 us-gaap_DeferredTaxAssetsOther |  |  |  | 23,500,000 us-gaap_DeferredTaxAssetsOther |  |  |  | 22,100,000 us-gaap_DeferredTaxAssetsOther | 23,500,000 us-gaap_DeferredTaxAssetsOther |
Total deferred tax assets | 190,100,000 us-gaap_DeferredTaxAssetsGross |  |  |  | 160,100,000 us-gaap_DeferredTaxAssetsGross |  |  |  | 190,100,000 us-gaap_DeferredTaxAssetsGross | 160,100,000 us-gaap_DeferredTaxAssetsGross |
Valuation allowance | (190,100,000)us-gaap_DeferredTaxAssetsValuationAllowance |  |  |  | (160,100,000)us-gaap_DeferredTaxAssetsValuationAllowance |  |  |  | (190,100,000)us-gaap_DeferredTaxAssetsValuationAllowance | (160,100,000)us-gaap_DeferredTaxAssetsValuationAllowance |
Net deferred tax assets | 0 us-gaap_DeferredTaxAssetsNet |  |  |  | 0 us-gaap_DeferredTaxAssetsNet |  |  |  | 0 us-gaap_DeferredTaxAssetsNet | 0 us-gaap_DeferredTaxAssetsNet |
Increase (decrease) in the valuation allowance |  |  |  |  |  |  |  |  | 30,000,000 us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount | (74,000,000)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount | (6,000,000)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Unrecognized Tax Benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Beginning Balance |  |  |  | 4,274,000 us-gaap_UnrecognizedTaxBenefits |  |  |  | 4,104,000 us-gaap_UnrecognizedTaxBenefits | 4,274,000 us-gaap_UnrecognizedTaxBenefits | 4,104,000 us-gaap_UnrecognizedTaxBenefits | 0 us-gaap_UnrecognizedTaxBenefits
Increase related to current year tax position |  |  |  |  |  |  |  |  | 720,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 164,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 49,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Increase related to prior year tax position |  |  |  |  |  |  |  |  | 509,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 6,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 4,055,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Ending Balance | 5,503,000 us-gaap_UnrecognizedTaxBenefits |  |  |  | 4,274,000 us-gaap_UnrecognizedTaxBenefits |  |  |  | 5,503,000 us-gaap_UnrecognizedTaxBenefits | 4,274,000 us-gaap_UnrecognizedTaxBenefits | 4,104,000 us-gaap_UnrecognizedTaxBenefits
Unrecognized Tax Benefits that Would Impact Effective Tax Rate | 4,000,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |  |  |  |  |  |  |  | 4,000,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |  |
Stock Option [Member] |  |  |  |  |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Operating Loss Carry-forwards | 5,200,000 us-gaap_OperatingLossCarryforwards / us-gaap_FinancialInstrumentAxis = us-gaap_StockOptionMember |  |  |  |  |  |  |  | 5,200,000 us-gaap_OperatingLossCarryforwards / us-gaap_FinancialInstrumentAxis = us-gaap_StockOptionMember |  |
State [Member] |  |  |  |  |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Operating Loss Carry-forwards | 132,300,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember |  |  |  |  |  |  |  | 132,300,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember |  |
California [Member] |  |  |  |  |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net operating loss carry-forward expired |  |  |  |  |  |  |  |  | 54,300,000 xoma_OperatingLossCarryforwardsExpirationDuringPeriod / us-gaap_IncomeTaxAuthorityAxis = stpr_CA | 16,800,000 xoma_OperatingLossCarryforwardsExpirationDuringPeriod / us-gaap_IncomeTaxAuthorityAxis = stpr_CA | 10,400,000 xoma_OperatingLossCarryforwardsExpirationDuringPeriod / us-gaap_IncomeTaxAuthorityAxis = stpr_CA
Federal [Member] |  |  |  |  |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Operating Loss Carry-forwards | 292,300,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember |  |  |  |  |  |  |  | 292,300,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember |  |
Net operating loss carry-forward expired |  |  |  |  |  |  |  |  | $ 0 xoma_OperatingLossCarryforwardsExpirationDuringPeriod / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember | $ 0 xoma_OperatingLossCarryforwardsExpirationDuringPeriod / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember | $ 0 xoma_OperatingLossCarryforwardsExpirationDuringPeriod / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember

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