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Royalty Purchase Agreements - Summary of Acquisition of Royalty Rights (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] |  |
Balance at beginning of period | $ 15,000 |
Balance at end of period | 34,375 | $ 15,000
Agenus |  |
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] |  |
Acquisition of royalty rights |  | $ 15,000
Bioasis Technologies Inc |  |
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] |  |
Acquisition of royalty rights | 375 |
Aronora |  |
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] |  |
Acquisition of royalty rights | 9,000 |
Palobiofarma, S.L. |  |
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] |  |
Acquisition of royalty rights | $ 10,000 |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Long-term royalty receivable non current. + References No definition available. + Details
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Namespace Prefix: | xoma_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Royalty rights acquired. + References No definition available. + Details
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Namespace Prefix: | xoma_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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- Details
Name: | srt_CounterpartyNameAxis=xoma_BioasisTechnologiesIncMember
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |