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Capital Stock - BVF Ownership (Details) - BVF [Member] | Sep. 30, 2021 | Feb. 29, 2020
--- | --- | ---
Series Y Convertible Preferred Stock [Member] |  |
Sale of stock |  |
Beneficial ownership limitation (as a percent) |  | 50.00%
Xoma Corporation |  |
Sale of stock |  |
Ownership interest (as a percent) | 31.20% |
Ownership interest, if shares are converted (as a percent) | 52.30% |

X
- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
Name: | us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition 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. + References No definition available. + Details
Name: | us-gaap_SubsidiarySaleOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the beneficial ownership limitation. + References No definition available. + Details
Name: | xoma_BeneficialOwnershipLimitation
Namespace Prefix: | xoma_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage, if shares are converted. + References No definition available. + Details
Name: | xoma_NoncontrollingInterestOwnershipPercentageByParentIfSharesConverted
Namespace Prefix: | xoma_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | dei_LegalEntityAxis=xoma_BiotechnologyValueFundLPMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=xoma_SeriesYConvertiblePreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_OwnershipAxis=xoma_XomaCorporationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |