Basis of Presentation and Significant Accounting Policies - Purchase of Rights to Future Milestones and Royalties (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Purchase of Rights to Future Milestones and Royalties
Impairment of long-term royalty receivable	$ 0.0	$ 0.0

The aggregate amount of write-downs for impairments recognized during the period for long term royalty receivable assets.

No definition available.

Name:	xoma_ImpairmentOfLongTermRoyaltyReceivable
Namespace Prefix:	xoma_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration