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Summary of Significant Accounting Policies (Details) (USD $) In Millions, except Share data, unless otherwise specified | 0 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Nov. 01, 2013 | Dec. 03, 2013 | Dec. 31, 2013 | Dec. 31, 2014 | Jun. 26, 2013
Fair value of financial instruments |  |  |  |  |
Assets recorded at fair value |  |  |  | $ 0 us-gaap_AssetsFairValueDisclosure |
Liabilities recorded at fair value |  |  |  | $ 0 us-gaap_LiabilitiesFairValueDisclosure |
Reverse Stock Split and Conversion of Preferred Stock |  |  |  |  |
Reverse stock split ratio | 3.1 us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 |  |  |  |
Series A-1 Convertible Preferred Stock |  |  |  |  |
Reverse Stock Split and Conversion of Preferred Stock |  |  |  |  |
Conversion of Stock, Shares Converted |  | 49,671,392 us-gaap_ConversionOfStockSharesConverted1 / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember | (49,671,392)us-gaap_ConversionOfStockSharesConverted1 / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember |  |
Exchange of Series A-1 and A-2 preferred for common stock (in shares) |  | 16,022,915 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember |  |  | 5,561,006 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember
Series A-2 Convertible Preferred Stock |  |  |  |  |
Reverse Stock Split and Conversion of Preferred Stock |  |  |  |  |
Conversion of Stock, Shares Converted |  | 1,851,814 us-gaap_ConversionOfStockSharesConverted1 / us-gaap_StatementClassOfStockAxis = xncr_SeriesA2ConvertiblePreferredStockMember | (1,851,814)us-gaap_ConversionOfStockSharesConverted1 / us-gaap_StatementClassOfStockAxis = xncr_SeriesA2ConvertiblePreferredStockMember |  |
Exchange of Series A-1 and A-2 preferred for common stock (in shares) |  | 597,359 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = xncr_SeriesA2ConvertiblePreferredStockMember |  |  | 1,851,814 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = xncr_SeriesA2ConvertiblePreferredStockMember

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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