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Lease Agreement - Leased facilities (Details) - Leased facilities, Emeryville, California | 6 Months Ended
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Jun. 30, 2022 facility
Leases |
Operating leases, number of leased facilities | 1
Option to extend lease | true

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- 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_LesseeLeaseDescriptionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Description of terms and conditions of option to extend lessee's operating lease. Includes, but is not limited to, information about option recognized as part of right-of-use asset and lease liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI https://asc.fasb.org/extlink&oid=128292326&loc=SL77918673-209980 + Details
Name: | us-gaap_LesseeOperatingLeaseOptionToExtend
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Operating leases, number of leased facilities. + References No definition available. + Details
Name: | xoma_OperatingLeasesNumberOfLeasedFacilities
Namespace Prefix: | xoma_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=xoma_LeasedFacilitiesEmeryvilleCaliforniaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |