Consolidated Statements of Convertible and Redeemable Convertible Preferred Stock and Stockholders Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Redeemable Convertible Preferred Stock [Member]	Series C Redeemable Convertible Preferred Stock [Member] Redeemable Convertible Preferred Stock [Member]	Series D Redeemable Convertible Preferred Stock [Member] Redeemable Convertible Preferred Stock [Member]	Subscription Receivable [Member]	Common Stock [Member]	Common Stock [Member] Series C and D Redeemable Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Series C and D Redeemable Convertible Preferred Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Stockholders Equity, Beginning Balance at Dec. 31, 2012	$ (22,250)								$ 465			$ (22,715)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2012							426,309
Temporary Equity, Beginning Balance at Dec. 31, 2012		$ 2,543	$ 17,172
Temporary Equity, Beginning Balance, Shares at Dec. 31, 2012		224,564	3,144,934
Issuance of common stock for exercise of stock options	26								26
Issuance of common stock for exercise of stock options, Shares							91,272
Issuance of common stock for milestone payment	57						$ 1		56
Issuance of common stock for milestone payment- shares							283,069
Stock-based compensation, value	11								11
Stock-based compensation, shares							35,353
Conversion of convertible securities, shares issued, value			$ 7,798
Conversion of convertible securities, shares issued			1,495,276
Issuance of Series C redeemable convertible preferred stock and warrants				$ 3,322
Issuance of Series C redeemable convertible preferred stock and warrants shares				647,830
Stock subscription receivable						$ (1,159)
Stock-based compensation	11								11
Adjustment to redemption value on redeemable convertible preferred stock	(5,713)		$ 5,713						(569)			(5,144)
Net loss	(6,832)											(6,832)
Stockholders Equity, Ending Balance at Dec. 31, 2013	(34,690)						$ 1					(34,691)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2013							836,003
Temporary Equity, Ending Balance at Dec. 31, 2013		$ 2,543	$ 34,005			(1,159)
Temporary Equity, Ending Balance, Shares at Dec. 31, 2013		224,564	5,288,040
Issuance of common stock for exercise of stock options	305						$ 1		304
Issuance of common stock for exercise of stock options, Shares							752,698
Repayment of stock subscription receivable						$ 1,159
Conversion of convertible securities, shares issued, value			$ 433
Conversion of convertible securities, shares issued			82,927
Stock-based compensation	302								302
Adjustment to redemption value on redeemable convertible preferred stock	(49,849)		$ 49,849						(606)			(49,243)
Other comprehensive loss	(10)										$ (10)
Issuance of stock, value				$ 1,365	$ 56,180
Issuance of stock, shares				261,737	11,409,360
Net loss	(23,873)											(23,873)
Stockholders Equity, Ending Balance at Dec. 31, 2014	(107,815)						$ 2				(10)	(107,807)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2014							1,588,701
Temporary Equity, Ending Balance at Dec. 31, 2014	$ 144,375	$ 2,543	$ 141,832
Temporary Equity, Ending Balance, Shares at Dec. 31, 2014	17,266,628	224,564	17,042,064
Issuance of common stock for exercise of stock options	$ 18								18
Issuance of common stock for exercise of stock options, Shares							41,505
Conversion of convertible securities, shares issued		(224,564)					252,817
Stock-based compensation	3,186								3,186
Vesting of early exercised stock options	90								90
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants			$ 8,976
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants, Shares			481,671
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants and reclassification of preferred stock warrant liability upon initial public offering			$ 11,785
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants and reclassification of preferred stock warrant liability upon initial public offering, Shares			390,032
Adjustment to redemption value on redeemable convertible preferred stock	(374,015)		$ 374,015						(895)			(373,120)
Conversion of preferred stock to common stock upon initial public offering	2,543	$ (2,543)							2,543
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	536,608		$ (536,608)				$ 18		536,590
Conversion of redeemable convertible preferred stock to common stock upon initial public offering, Shares			(17,913,767)				18,109,136
Issuance of stock, value	143,549						$ 9		143,540
Issuance of stock, shares							9,315,000
Redeemable convertible preferred stock dividend	(5,543)								(5,543)
Redeemable convertible preferred stock dividend, common stock issued								$ 1		$ (1)
Redeemable convertible preferred stock dividend								750,946
Reclassification of other-than-temporary losses on short-term investments to net loss	10										$ 10
Net loss	(53,260)											(53,260)
Stockholders Equity, Ending Balance at Dec. 31, 2015	$ 145,371						$ 30		$ 679,528			$ (534,187)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2015							30,058,105