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Summary of Significant Accounting Policies (Details 6) (USD $)	3 Months Ended								12 Months Ended			1 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2014	Dec. 31, 2014
Revenue recognition
Total	$ 963,300,000xncr_RevenueRecognitionMilestoneMethodPending								$ 963,300,000xncr_RevenueRecognitionMilestoneMethodPending				$ 963,300,000xncr_RevenueRecognitionMilestoneMethodPending
Revenue recognized	5,664,000xncr_CollaborationsLicensesAndMilestonesRevenue	848,000xncr_CollaborationsLicensesAndMilestonesRevenue	824,000xncr_CollaborationsLicensesAndMilestonesRevenue	2,184,000xncr_CollaborationsLicensesAndMilestonesRevenue	1,745,000xncr_CollaborationsLicensesAndMilestonesRevenue	3,161,000xncr_CollaborationsLicensesAndMilestonesRevenue	3,921,000xncr_CollaborationsLicensesAndMilestonesRevenue	1,345,000xncr_CollaborationsLicensesAndMilestonesRevenue	9,520,000xncr_CollaborationsLicensesAndMilestonesRevenue	10,172,000xncr_CollaborationsLicensesAndMilestonesRevenue	9,524,000xncr_CollaborationsLicensesAndMilestonesRevenue
Accounts receivable	2,966,000us-gaap_AccountsReceivableNetCurrent				59,000us-gaap_AccountsReceivableNetCurrent				2,966,000us-gaap_AccountsReceivableNetCurrent	59,000us-gaap_AccountsReceivableNetCurrent			2,966,000us-gaap_AccountsReceivableNetCurrent
Development-based
Revenue recognition
Total	181,800,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								181,800,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				181,800,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
Regulatory-based
Revenue recognition
Total	426,000,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember								426,000,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember				426,000,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember
Sales-based
Revenue recognition
Total	355,500,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember								355,500,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember				355,500,000xncr_RevenueRecognitionMilestoneMethodPending / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember
UNITED STATES
Revenue recognition
Revenue recognized									8,600,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_StatementGeographicalAxis = country_US	4,200,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_StatementGeographicalAxis = country_US	4,400,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_StatementGeographicalAxis = country_US
Non United States [Member]
Revenue recognition
Revenue recognized									900,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_StatementGeographicalAxis = xncr_NonUnitedStatesMember	6,000,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_StatementGeographicalAxis = xncr_NonUnitedStatesMember	5,100,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_StatementGeographicalAxis = xncr_NonUnitedStatesMember
Amgen, Inc.
Revenue recognition
Revenue recognized									6,900,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_AmgenIncMember	2,200,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_AmgenIncMember	1,800,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_AmgenIncMember	5,200,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_AmgenIncMember
MorphoSys
Revenue recognition
Total	299,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember								299,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember				299,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember
Revenue recognized									0xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember	3,000,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember	2,000,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember
Accounts receivable					100,000us-gaap_AccountsReceivableNetCurrent / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember					100,000us-gaap_AccountsReceivableNetCurrent / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember
MorphoSys | Development-based
Revenue recognition
Total	62,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								62,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				62,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
MorphoSys | Regulatory-based
Revenue recognition
Total	187,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember								187,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember				187,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember
MorphoSys | Sales-based
Revenue recognition
Total	50,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember								50,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember				50,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MorphoSysMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember
Alexion Pharmaceuticals, Inc.
Revenue recognition
Total	399,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember								399,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember				399,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember
Alexion Pharmaceuticals, Inc. | Development-based
Revenue recognition
Total	51,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								51,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				51,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
Alexion Pharmaceuticals, Inc. | Regulatory-based
Revenue recognition
Total	168,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember								168,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember				168,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember
Alexion Pharmaceuticals, Inc. | Sales-based
Revenue recognition
Total	180,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember								180,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember				180,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_AlexionPharmaceuticalsIncMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember
BI
Revenue recognition
Total	27,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember								27,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember				27,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember
BI | Development-based
Revenue recognition
Total	9,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								9,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				9,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
BI | Regulatory-based
Revenue recognition
Total	6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember								6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember				6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember
BI | Sales-based
Revenue recognition
Total	12,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember								12,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember				12,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_BIMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember
Merck Sharp & Dohme Corp.
Revenue recognition
Total	9,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember								9,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember				9,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember
Revenue recognized				500,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember					600,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember	1,000,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember
Merck Sharp & Dohme Corp. | Development-based
Revenue recognition
Total	3,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								3,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				3,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
Merck Sharp & Dohme Corp. | Regulatory-based
Revenue recognition
Total	6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember								6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember				6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_MerckSharpAndDohmeCorpMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember
Novo Nordisk
Revenue recognition
Total	167,300,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember								167,300,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember				167,300,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember
Revenue recognized													100,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember
Accounts receivable	3,000,000us-gaap_AccountsReceivableNetCurrent / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember								3,000,000us-gaap_AccountsReceivableNetCurrent / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember				3,000,000us-gaap_AccountsReceivableNetCurrent / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember
Novo Nordisk | Development-based
Revenue recognition
Total	36,300,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								36,300,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				36,300,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
Novo Nordisk | Regulatory-based
Revenue recognition
Total	51,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember								51,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember				51,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember
Novo Nordisk | Sales-based
Revenue recognition
Total	80,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember								80,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember				80,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_NovoNordiskMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember
Janssen
Revenue recognition
Total	10,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember								10,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember				10,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember
Revenue recognized									0xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_JanssenMember	0xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_JanssenMember	1,400,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_JanssenMember
Janssen | Development-based
Revenue recognition
Total	6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember								6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember				6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember
Janssen | Sales-based
Revenue recognition
Total	4,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember								4,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember				4,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_JanssenMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember
2009 Research License and Commercialization Agreement | CSL Limited
Revenue recognition
Total	15,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember								15,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember				15,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember
Revenue recognized											1,800,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember
2009 Research License and Commercialization Agreement | CSL Limited | Development-based
Revenue recognition
Total	6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember								6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember				6,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember
2009 Research License and Commercialization Agreement | CSL Limited | Regulatory-based
Revenue recognition
Total	4,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember								4,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember				4,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_RegulatoryBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember
2009 Research License and Commercialization Agreement | CSL Limited | Sales-based
Revenue recognition
Total	5,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember								5,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember				5,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_ResearchLicenseAndCommercialization2009AgreementMember
May 2013 Research License and Commercialization Agreement | CSL Limited
Revenue recognition
Total	36,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember								36,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember				36,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember
Revenue recognized									700,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember	2,400,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember	1,800,000xncr_CollaborationsLicensesAndMilestonesRevenue / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember
May 2013 Research License and Commercialization Agreement | CSL Limited | Development-based
Revenue recognition
Total	8,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember								8,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember				8,000,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_DevelopmentBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember
May 2013 Research License and Commercialization Agreement | CSL Limited | Regulatory-based
Revenue recognition
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May 2013 Research License and Commercialization Agreement | CSL Limited | Sales-based
Revenue recognition
Total	$ 24,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember								$ 24,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember				$ 24,500,000xncr_RevenueRecognitionMilestoneMethodPending / us-gaap_CounterpartyNameAxis = xncr_CSLLimitedMember / xncr_PotentialMilestonesTypeAxis = xncr_SalesBasedMilestonesMember / us-gaap_TypeOfArrangementAxis = xncr_May2013ResearchLicenseAndCommercializationAgreementMember

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