Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders Equity (Deficit) - USD ($) $ in Thousands	Series A-1	Series A-2	Series B	Series C	Series D	Series E	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2013	$ 874	$ 7,724	$ 15,464	$ 19,100	$ 27,357	$ 42,294
Balance (in shares) at Dec. 31, 2013	282,849	1,703,959	3,249,877	4,055,125	5,054,945	6,930,967
Increase (Decrease) in Redeemable Convertible Preferred Stock
Accretion of redeemable convertible preferred stock to redemption value	$ 26	$ 240	$ 520	$ 722	$ 1,115	$ 1,947
Conversion of redeemable convertible preferred stock into common stock	$ (900)	$ (7,964)	$ (15,984)	$ (19,822)	$ (28,472)	$ (44,241)
Conversion of redeemable convertible preferred stock into common stock (in shares)	(282,849)	(1,703,959)	(3,249,877)	(4,055,125)	(5,054,945)	(6,930,967)
Balance at Dec. 31, 2013							$ 1		$ (89,544)	$ (89,543)
Balance (in shares) at Dec. 31, 2013							1,411,986
Increase (Decrease) in Stockholders' Equity
Accretion of redeemable convertible preferred stock to redemption value								$ (880)	(3,690)	(4,570)
Stock-based compensation expense								1,653		1,653
Conversion of redeemable convertible preferred stock into common stock							$ 13	96,341	21,029	117,383
Conversion of redeemable convertible preferred stock into common stock (in shares)							12,516,298
Issuance of common stock upon net settlement of warrants to purchase redeemable convertible preferred stock								1,226		1,226
Issuance of common stock upon net settlement of warrants to purchase redeemable convertible preferred stock (in shares)							68,700
Exercise of stock options								153		153
Exercise of stock options (in shares)							64,661
Issuance of common stock							$ 6	58,083		58,089
Issuance of common stock (in shares)							5,980,000
Net loss									(31,390)	(31,390)
Balance at Dec. 31, 2014							$ 20	156,576	(103,595)	53,001
Balance (in shares) at Dec. 31, 2014							20,041,645
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense								4,168		4,168
Issuance of common stock from exercise of stock options and employee stock purchase plan								1,804		1,804
Issuance of common stock from exercises of stock options and employee stock purchase plan (in shares)							442,687
Issuance of common stock							$ 4	33,252		33,256
Issuance of common stock (in shares)							3,691,049
Net loss									(45,290)	(45,290)
Balance at Dec. 31, 2015							$ 24	195,800	(148,885)	$ 46,939
Balance (in shares) at Dec. 31, 2015							24,175,381			24,175,381
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense								4,848		$ 4,848
Offering costs on issuance of common stock from secondary public offering								(215)		(215)
Issuance of common stock from exercise of stock options and employee stock purchase plan								1,018		1,018
Issuance of common stock from exercises of stock options and employee stock purchase plan (in shares)							251,990
Issuance of common stock							$ 6	39,717		39,723
Issuance of common stock (in shares)							6,055,341
Issuance of warrants								1,829		1,829
Net loss									(54,804)	(54,804)
Balance at Dec. 31, 2016							$ 30	$ 242,997	$ (203,689)	$ 39,338
Balance (in shares) at Dec. 31, 2016							30,482,712			30,482,712