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Commitments and Contingencies - Litigation (Details) - USD ($) | Sep. 27, 2016 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Loss Contingencies [Line Items] |  |  |
Loss contingency accrual |  | $ 0 | $ 2,355,000
DePinto vs John S Stafford Litigation Case |  |  |
Loss Contingencies [Line Items] |  |  |
Paid legal award | $ 2,375,000 |  |
Amount paid not covered by insurance | $ 0 |  |

X
- Definition Amount awarded to other party in judgment or settlement of litigation. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Definition Amount of loss contingency liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 + Details
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Balance Type: | credit
Period Type: | instant

X
- Definition The amount paid relating to a litigation settlement that is not covered by insurance. + References No definition available. + Details
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Namespace Prefix: | xncr_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |