Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net loss	$ (16,422)xncr_NetProfitLoss	$ (60,259)xncr_NetProfitLoss	$ (8,594)xncr_NetProfitLoss
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	882	711	527
Stock-based compensation	1,861us-gaap_ShareBasedCompensation	198us-gaap_ShareBasedCompensation	29us-gaap_ShareBasedCompensation
Abandonment of capitalized intangible assets	509us-gaap_NoncashProjectAbandonmentCosts	205us-gaap_NoncashProjectAbandonmentCosts	388us-gaap_NoncashProjectAbandonmentCosts
Gain from non-monetary exchange			(754)xncr_GainLossOnNonMonetaryExchange
Gain on disposal of assets	(2)us-gaap_GainLossOnDispositionOfAssets1	(16)us-gaap_GainLossOnDispositionOfAssets1	(86)us-gaap_GainLossOnDispositionOfAssets1
Loss on exchange of notes for preferred stock		48,556
Accrued interest on convertible promissory notes (See Note 3)		1,211us-gaap_AmortizationOfFinancingCosts	2,456us-gaap_AmortizationOfFinancingCosts
Changes in operating assets and liabilities:
Accounts receivable	(2,907)	293	(325)
Prepaid expenses and other assets	(73)	115	(90)
Other assets	41	(20)	15
Accounts payable	(943)	1,319	(522)
Accrued expenses	859	109	460
Deferred revenue	(5,156)	2,125	(4,556)
Net cash used in operating activities	(21,351)	(5,453)	(11,052)
Cash flows from investing activities
Purchase of intangible assets	(1,505)	(1,158)	(1,217)
Purchase of property and equipment	(780)	(136)	(41)
Proceeds from sale of property and equipment	2	16	97
Net cash used in investing activities	(2,283)	(1,278)	(1,161)
Cash flows from financing activities
Preferred stock issuance cost		(116)us-gaap_PaymentsOfStockIssuanceCosts
Proceeds from the sale of Series A-1 preferred		10,000
Proceeds from issuance of common stock upon exercise of stock awards	9	13
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	307us-gaap_ProceedsFromStockPlans
Proceeds from sale of common stock		80,517
Payments of Initial Public Offering costs		(8,010)
Payments on capital lease obligations	(8)	(10)	(12)
Net cash provided by financing activities	308	82,394	(12)
Net increase (decrease) in cash	(23,326)	75,663	(12,225)
Cash, beginning of period	77,975	2,312	14,537
Cash, end of period	54,649	77,975	2,312
Cash paid for:
Interest	9us-gaap_InterestPaidNet	1us-gaap_InterestPaidNet	3us-gaap_InterestPaidNet
Taxes	1us-gaap_IncomeTaxesPaid
Supplemental Schedule of Noncash Investing and Financing Activities
Capitalization of licensing rights acquired in non-monetary exchange			754us-gaap_FairValueOfAssetsAcquired
Equipment acquired under capital lease			22us-gaap_CapitalLeaseObligationsIncurred
Settlement of notes payable for preferred stock		$ 22,134us-gaap_NotesIssued1

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- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the gain loss on non monetary exchange.

+ References

No definition available.

+ Details

Name:	xncr_GainLossOnNonMonetaryExchange
Namespace Prefix:	xncr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the increase (decrease) during the reporting period in amount due within one year from a collaboration agreement, defined as a contractual agreement between the company and another entity(referred to as a partner) from which the company is entitled to receive contractual payments.

+ References

No definition available.

+ Details

Name:	xncr_IncreaseDecreaseCollaborationPartnerReceivable
Namespace Prefix:	xncr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the profit or loss for the period, net of income taxes.

+ References

No definition available.

+ Details

Name:	xncr_NetProfitLoss
Namespace Prefix:	xncr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow for cost incurred in the payment of initial public offering costs

+ References

No definition available.

+ Details

Name:	xncr_PaymentsOfInitialPublicOfferingCosts
Namespace Prefix:	xncr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	xncr_SupplementalCashFlowInformationCashPaidAbstract
Namespace Prefix:	xncr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration