Description of Business (Details)	Mar. 31, 2013 Antibody
Description of Business [Abstract]
Number classes of preclinical antibodies	3

Number of classes of preclinical antibodies for the treatment of diabetes.

No definition available.

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