Legal Proceedings, Commitments and Contingencies - Additional Information (Details) $ in Millions	Jun. 30, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Estimate of milestone payments	$ 15.5

Reflects estimated obligations that are due and payable upon achievement of certain criteria categorized as developmental, regulatory, or commercial milestones.

No definition available.

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