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Income Taxes (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Income Taxes [Abstract] |  |  |  |
Income tax benefit - Refundable Credits | $ 0.1 | $ 0 | $ 0.1 | $ 0

X
- Details
Name: | us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The income tax benefit relating to refundable credits. + References No definition available. + Details
Name: | xoma_IncomeTaxBenefit-RefundableCredits
Namespace Prefix: | xoma_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration