Commitments and Contingencies - Collaborative Agreements, Royalties and Milestone Payments (Details) $ in Millions	Dec. 31, 2020 USD ($) product
Commitments and Contingencies
Estimate of milestone payments \| $	$ 7.6
Assumed number of products per contract \| product	1

Estimated potential obligations that come due and payable upon achievement of developmental, regulatory, or commercial milestones by company's licensees. Represent contingent payments not recorded on balance sheet.

No definition available.

Name:	xoma_ContractualObligationEstimatedMilestonePayments
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant