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Basis of Presentation and Significant Accounting Policies - Use of Estimates and Purchase of Rights to Future Milestones and Royalties (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
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Sep. 30, 2020 | Dec. 31, 2019
Purchase of Rights to Future Milestones and Royalties |  |
Impairment of long-term royalty receivable | $ 0.0 | $ 0.0

X
- Definition The aggregate amount of write-downs for impairments recognized during the period for long term royalty receivable assets. + References No definition available. + Details
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Namespace Prefix: | xoma_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | xoma_PurchaseOfRightsToFutureMilestonesAndRoyaltiesAbstract
Namespace Prefix: | xoma_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration