Statements of Mezzanine Equity and Stockholders’ Equity (Deficit) (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2011	$ 1	$ 1,014	$ (158,718)	$ (157,703)us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2011	72,302
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss			(8,594)	(8,594)xncr_NetProfitLoss
Stock-based compensation expense		29		29
Balance at Dec. 31, 2012	1	1,043	(167,312)	(166,268)us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2012	72,302
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Deemed contribution on exchange of Series A-E Preferred Stock for Series A-1		143,681		143,681
Deemed contribution on exchange of Series A-1 preferred for Series A-2 preferred stock		7,489		7,489
Deemed dividend on sale of Series A-1 preferred		(3,429)		(3,429)
Exchange of Series A-1 and A-2 preferred for common stock	166	79,435		79,601
Exchange of Series A-1 and A-2 preferred for common stock (in shares)	16,620,274
Sale of common stock, net of issuance cost	146	72,361		72,507
Sale of common stock, net of issuance cost (in shares)	14,639,500
Issuance of common stock upon exercise and vesting of stock awards	1	12		13
Issuance of common stock upon exercise and vesting of stock awards (in shares)	22,391
Net loss			(60,259)	(60,259)xncr_NetProfitLoss
Stock-based compensation expense		198		198
Balance at Dec. 31, 2013	314	300,790	(227,571)	73,533us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2013	31,354,467
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise and vesting of stock awards		11		11
Issuance of common stock upon exercise and vesting of stock awards (in shares)	15,941
Issuance of common stock under the Employee Stock Purchase Plan		307		307
Issuance of common stock under the Employee Stock Purchase Plan (in shares)	63,864
Net loss			(16,422)	(16,422)xncr_NetProfitLoss
Stock-based compensation expense		1,861		1,861
Balance at Dec. 31, 2014	$ 314	$ 302,969	$ (243,993)	$ 59,290us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2014	31,434,272

This element represents the amount of recognized equity-based compensation during the period, that is, the amount recognized as expense in the income statement (or as asset if compensation is capitalized). Alternate captions include the words "stock-based compensation".

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 35

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415241&loc=d3e4534-113899

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 20

-Section 55

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11149-113907

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 20

-Section 55

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11178-113907