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Concentration of Risk, Segment and Geographic Information - Additional Information 2 (Details) | 12 Months Ended
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Dec. 31, 2017 Segment
Concentration Of Risk Segment And Geographic Information [Abstract] |
Number of operating segments | 1

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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Concentration of risk segment and geographic information. + References No definition available. + Details
Name: | xoma_ConcentrationOfRiskSegmentAndGeographicInformationAbstract
Namespace Prefix: | xoma_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration