Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Total
Balance at Dec. 31, 2013	$ 47,898	$ 32,248
Balance (in shares) at Dec. 31, 2013	68,837,703	34,129,571
Balance at Dec. 31, 2014	$ 47,898	$ 32,248
Balance (in shares) at Dec. 31, 2014	68,837,703	34,129,571
Balance at Dec. 31, 2013				$ 1,312	$ (53,496)		$ (52,184)
Balance (in shares) at Dec. 31, 2013			393,346
Increase (Decrease) in Stockholders' Equity (Deficit)
Proceeds from exercises of common stock options			$ 1	117			118
Proceeds from exercises of common stock options (in shares)			99,854
Stock-based compensation expense				897			897
Net loss					(21,884)		(21,884)
Balance at Dec. 31, 2014			$ 1	2,326	(75,380)		(73,053)
Balance (in shares) at Dec. 31, 2014			493,200
Increase (decrease) in Convertible Preferred Stock
Issuance of series B convertible preferred stock, net of issuance cost of $0.1 million		$ 12,331
Issuance of series B convertible preferred stock, net of issuance cost of $0.1 million (in shares)		13,062,965
Conversion of convertible preferred stock into common stock	$ (47,898)	$ (44,579)
Conversion of convertible preferred stock into common stock (in shares)	(68,837,703)	(47,192,536)
Increase (Decrease) in Stockholders' Equity (Deficit)
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $1.1 million and $7.3 million for the years 2016 and 2015 respectively			$ 5	61,739			61,744
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $1.1 million and $7.3 million for the years 2016 and 2015 respectively (in shares)			5,750,000
Conversion of convertible preferred stock into common stock			$ 9	92,468			92,477
Conversion of convertible preferred stock into common stock (in shares)			9,029,549
Proceeds from exercises of common stock options				91			91
Proceeds from exercises of common stock options (in shares)			40,548
Stock-based compensation expense				1,658			1,658
Conversion of series B preferred stock warrants into warrants for the purchase of common stock				206			206
Net loss					(32,630)		(32,630)
Balance at Dec. 31, 2015			$ 15	158,488	(108,010)		$ 50,493
Balance (in shares) at Dec. 31, 2015			15,313,297				15,313,297
Increase (Decrease) in Stockholders' Equity (Deficit)
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $1.1 million and $7.3 million for the years 2016 and 2015 respectively			$ 3	11,991			$ 11,994
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $1.1 million and $7.3 million for the years 2016 and 2015 respectively (in shares)			3,243,015
Proceeds from exercises of common stock options (in shares)							230,019
Proceeds from exercises of common stock options and warrants			$ 1	502			$ 503
Proceeds from exercises of common stock options and warrants (in shares)			261,260
Stock-based compensation expense				2,160			2,160
Unrealized losses on short-term investments						$ (4)	(4)
Net loss					(36,060)		(36,060)
Balance at Dec. 31, 2016			$ 19	$ 173,141	$ (144,070)	$ (4)	$ 29,086
Balance (in shares) at Dec. 31, 2016			18,817,572				18,817,572