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Description of Business - Additional Information (Details) $ in Millions | Jun. 30, 2018 USD ($)
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Description Of Business [Abstract] |
Cash | $ 38.7

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_Cash
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Description of business. + References No definition available. + Details
Name: | xoma_DescriptionOfBusinessAbstract
Namespace Prefix: | xoma_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration