Consolidated Statements of Changes in Mandatorily Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Preferred Stock [Member]	Series B Preferred Stock [Member]	Series C Preferred Stock [Member]	Series D Preferred Stock [Member]	Series E Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Balances at beginning of period at Dec. 31, 2012	$ (93,760)									$ (93,760)
Beginning Balance at Dec. 31, 2012		$ 15,605	$ 13,865	$ 38,592	$ 20,323	$ 15,237
Balances at beginning of period, shares at Dec. 31, 2012							411,226
Balance, Shares at Dec. 31, 2012		557,339	515,836	3,551,060	2,430,054	1,063,951
Accretion of preferred stock	(4,653)							$ (862)		(3,791)
Accretion of preferred stock		$ 683	$ 577	$ 1,604	$ 1,140	$ 649
Issuance of common stock from initial public offering, shares		0					5,400,000
Issuance of common stock from initial public offering, value	$ 46,820							46,820
Conversion of preferred stock into common stock at initial public offering, shares	8,631,427	(557,339)	(515,836)	(3,551,060)	(2,430,054)	(1,063,951)	8,631,427
Conversion of preferred stock to common stock at initial public offering, value	$ 108,275	$ (16,288)	$ (14,442)	$ (40,196)	$ (21,463)	$ (15,886)	$ 1	108,274
Conversion of warrants from warrants for preferred stock to warrants for common stock	2,514							2,514
Exercise of stock options, shares							177,165
Exercise of stock options	387							387
Stock-based compensation	1,145							1,145
Net loss	(29,281)									(29,281)
Other comprehensive income	22								$ 22
Balances at end of period at Dec. 31, 2013	31,469						$ 1	158,278	22	(126,832)
Balances at end of period, shares at Dec. 31, 2013							14,619,818
Issuance of common stock from initial public offering, shares							3,318,917
Issuance of common stock from initial public offering, value	56,987						$ 1	56,986
Issuance of common stock for employee stock purchase plan, Shares							141,386
Issuance of common stock for employee stock purchase plan, Value	987							987
Exercise of stock options, shares							164,394
Exercise of stock options	411							411
Exercise of common stock warrant, shares							27,269
Exercise of common stock warrant, value	136							136
Stock-based compensation	4,926							4,926
Net loss	(50,038)									(50,038)
Other comprehensive income	(65)								(65)
Balances at end of period at Dec. 31, 2014	$ 44,813						$ 2	221,724	(43)	(176,870)
Balances at end of period, shares at Dec. 31, 2014	18,272,000						18,271,784
Issuance of common stock from initial public offering, shares		0	0	0	0	0	960,400
Issuance of common stock from initial public offering, value	$ 12,518	$ 0	$ 0	$ 0	$ 0	$ 0		12,518
Issuance of common stock for employee stock purchase plan, Shares		0	0	0	0	0	136,078
Issuance of common stock for employee stock purchase plan, Value	$ 1,295	$ 0	$ 0	$ 0	$ 0	$ 0		1,295
Exercise of stock options, shares	201,622						201,622
Exercise of stock options	$ 876	0	0	0	0	0		876
Exercise of common stock warrant, shares							250
Exercise of common stock warrants, net		0	0	0	0	0
Exercise of common stock warrant, value	2							2
Stock-based compensation	6,278	0	0	0	0	0	$ 0	6,278	0	0
Net loss	(45,581)	0	0	0	0	0	0	0	0	(45,581)
Other comprehensive income	14	0	0	0	0	0	0	0	14	0
Balances at end of period at Dec. 31, 2015	$ 20,215						$ 2	$ 242,693	$ (29)	$ (222,451)
Ending Balance at Dec. 31, 2015		$ 0	$ 0	$ 0	$ 0	$ 0
Balances at end of period, shares at Dec. 31, 2015	19,570,000						19,570,134
Balance, Shares at Dec. 31, 2015		0	0	0	0	0