Description of Business (Details)	Jun. 30, 2012
Description of Business [Abstract]
Number classes of preclinical antibodies	3
Number of product candidates on rights to develop and commercialize	1
Number of product candidates on option to develop and commercialize	2

The number of product candidates the entity has option to develop and commercialize.

No definition available.

Name:	xoma_NumberOfProductCandidatesOptions
Namespace Prefix:	xoma_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant