STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Stockholder Notes	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series B-1	Redeemable Convertible Preferred Stock Series C	Redeemable Convertible Preferred Stock Series D	Convertible Preferred Stock
Temporary equity, beginning balance at Dec. 31, 2012							$ 40,493				$ 474
Temporary equity, beginning balance, shares at Dec. 31, 2012							11,995,481				244,782
Beginning balance at Dec. 31, 2012	$ (26,045)		$ (393)			$ (25,652)
Beginning balance, shares at Dec. 31, 2012		779,989
Common stock issued in connection with a license agreement	199	$ 1		$ 198
Common stock issued in connection with a license agreement, shares		157,332
Exercise of stock options	$ 60			60
Exercise of stock options, shares	53,193	53,193
Interest on stockholder notes	$ (6)		(6)
Vesting of early exercise stock option	64			64
Stock-based compensation	343			343
Accretion to redemption value and cumulative dividends on preferred stock							$ 3,751
Accretion to redemption value and cumulative dividends on preferred stock	(3,751)			(665)		(3,086)
Net loss	(15,143)					(15,143)
Temporary equity, ending balance at Dec. 31, 2013							$ 44,244				$ 474
Temporary equity, ending balance, shares at Dec. 31, 2013							11,995,481				244,782
Ending balance at Dec. 31, 2013	(44,279)	$ 1	(399)			(43,881)
Ending balance, shares at Dec. 31, 2013		990,514
Issuance of preferred stock, net of issuance costs								$ 11,618	$ 15,808
Issuance of preferred stock, net of issuance costs, shares								3,355,107	3,107,701
Exercise of stock options	$ 8			8
Exercise of stock options, shares	6,006	6,006
Interest on stockholder notes	$ (5)		(5)
Vesting of early exercise stock option	58			58
Stock-based compensation	553			553
Accretion to redemption value and cumulative dividends on preferred stock							$ 4,566
Accretion to redemption value and cumulative dividends on preferred stock	(4,566)			(619)		(3,947)
Net loss	$ (30,310)					(30,310)
Temporary equity, ending balance at Dec. 31, 2014							$ 76,236				$ 474
Temporary equity, ending balance, shares at Dec. 31, 2014	18,703,071						18,458,289				244,782
Ending balance at Dec. 31, 2014	$ (78,541)	$ 1	(404)			(78,138)
Ending balance, shares at Dec. 31, 2014		996,520
Issuance of preferred stock, net of issuance costs									$ 4,969	$ 69,744
Issuance of preferred stock, net of issuance costs, shares									941,842	7,490,540
Issuance of preferred stock upon net exercise of warrants, shares								60,640
Conversion of redeemable convertible preferred stock to common stock in connection with initial public offering	159,163			159,163			$ (159,163)
Conversion of redeemable convertible preferred stock to common stock in connection with initial public offering, shares		26,951,311					(26,951,311)
Conversion of convertible preferred stock to common stock in connection with initial public offering	474			474							$ (474)
Conversion of convertible preferred stock to common stock in connection with initial public offering, shares		244,782									(244,782)
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796	81,764			81,764
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796, share		7,666,667
Extinguishment of preferred stock liability							$ 1,509
Extinguishment of preferred stock warrant liability	788			788
Exercise of stock options	$ 263			263
Exercise of stock options, shares	173,929	173,929
Interest on stockholder notes	$ (4)		(4)
Repayment on stockholders note	330		330
Stock-based compensation	3,986			3,986
Accretion to redemption value and cumulative dividends on preferred stock							$ 6,705
Accretion to redemption value and cumulative dividends on preferred stock	(6,705)			(2,751)		(3,954)
Other comprehensive loss	(76)				$ (76)
Net loss	$ (35,374)					(35,374)
Temporary equity, ending balance, shares at Dec. 31, 2015	0						0				0
Ending balance at Dec. 31, 2015	$ 126,068	$ 1	$ (78)	$ 243,687	$ (76)	$ (117,466)
Ending balance, shares at Dec. 31, 2015		36,033,209