Name:	us-gaap_AccountsAndOtherReceivablesNetCurrent
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479402/808-10-50-1

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Number of drug candidates under the terms of the agreement.

No definition available.

Name:	xoma_AgreementDrugCandidatesNumber
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Data Type:	xbrli:integerItemType
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Amount of income that is not accounted for under Topic 606, in accordance with the cost recovery method.

No definition available.

Name:	xoma_IncomeNotFromContractWithCustomerCostRecoveryMethod
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Amount of maximum consideration retained by Company excluded from maximum potential milestones under terms of agreement.

No definition available.

Name:	xoma_MaximumConsiderationRetained
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Data Type:	xbrli:monetaryItemType
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Maximum amount of potential milestones, potential royalties on sales, and other payments, excluding consideration retained by company under the terms of the agreement. Excludes regulatory and commercial milestones.

No definition available.

Name:	xoma_MilestonesReceivableExcludingConsiderationRetainedMaximum
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of cash outflow for consideration under agreements, including Royalty Purchase Agreements (RPAs), Assignment and Assumption Agreements (AAAs), Commercial payment purchase agreement (CPPAs) and other agreements classified as investing activities. Payments may include upfront payments, one-time payments, milestone payments and contingent consideration payments.

No definition available.

Name:	xoma_PaymentOfConsiderationUnderAgreementsInvestingActivities
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Maximum amount of potential regulatory and commercial milestones receivable under the terms of the agreement.

No definition available.

Name:	xoma_RegulatoryAndCommercialMilestonesReceivableMaximum
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Royalty and commercial payment receivables under cost recovery method probable and reasonably estimable to be received after one year or beyond the normal operating cycle, if longer.

No definition available.