Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Initial Public Offering (IPO)	Common Stock	Common Stock Initial Public Offering (IPO)	Additional Paid-in Capital	Additional Paid-in Capital Initial Public Offering (IPO)	Accumulated Deficit	Series A Convertible Preferred Stock Preferred Stock	Series B Convertible Preferred Stock Preferred Stock
Beginning balance at Dec. 31, 2012	$ (39,296)		$ 1		$ 272		$ (39,569)	$ 45,035	$ 0
Beginning balance (in shares) at Dec. 31, 2012			1,205,038					45,250,000	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of convertible preferred stock and initial public offering, net of issuance costs	0								$ 7,439
Issuance of convertible preferred stock and initial public offering, net of issuance costs (in shares)									5,142,859
Exercise of stock options and vesting of restricted stock awards	35		$ 1		34
Exercise of stock awards and vesting of restricted stock awards (shares)			286,837
Issuance of convertible preferred stock (in shares)									2,060,986
Issuance of series B convertible preferred stock upon the conversion of notes payable	0								$ 4,204
Beneficial conversion feature of Series B preferred stock discount	163				163				(163)
Accretion of Series B preferred stock discount	(163)				(163)				163
Issuance of common stock warrants in connection with notes payable	1,685				1,685
Exercise of stock warrants	9				9
Exercise of stock warrants (in shares)			144,262
Conversion of preferred stock warrant to common stock warrant	0
Stock-based compensation expense	1,260				1,260
Net loss	(18,025)						(18,025)
Ending balance at Dec. 31, 2013	(54,332)		$ 2		3,260		(57,594)	$ 45,035	$ 11,643
Ending balance (in shares) at Dec. 31, 2013			1,636,137					45,250,000	7,203,845
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs of $819,000								$ (45,035)	$ (11,643)
Issuance of convertible preferred stock and initial public offering, net of issuance costs	15,419	$ 106,882	$ 2	$ 14	15,417	$ 106,868
Issuance of convertible preferred stock and initial public offering, net of issuance costs (in shares)			1,743,680	14,010,424
Exercise of stock options and vesting of restricted stock awards	65				65
Exercise of stock awards and vesting of restricted stock awards (shares)			190,701
Issuance of convertible preferred stock (in shares)								(45,250,000)	(7,203,845)
Issuance of common stock warrants in connection with notes payable	254				254
Exercise of stock warrants	15				15
Exercise of stock warrants (in shares)			352,318
Conversion of preferred stock warrant to common stock warrant	247				247
Stock-based compensation expense	2,432				2,432
Net loss	(34,156)						(34,156)
Ending balance at Dec. 31, 2014	36,826		$ 18		128,558		(91,750)	$ 0	$ 0
Ending balance (in shares) at Dec. 31, 2014			17,933,260					0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of convertible preferred stock and initial public offering, net of issuance costs	12,650			$ 2		$ 12,648
Issuance of convertible preferred stock and initial public offering, net of issuance costs (in shares)			1,446,781	1,446,781
Exercise of stock options and vesting of restricted stock awards	$ 63				63
Exercise of stock awards and vesting of restricted stock awards (shares)	78,585		239,083
Issuance of common stock warrants in connection with notes payable	$ 328				328
Conversion of preferred stock warrant to common stock warrant	0
Stock-based compensation expense	2,529				2,529
Net loss	(33,452)						(33,452)
Ending balance at Dec. 31, 2015	$ 18,944		$ 20		$ 144,126		$ (125,202)	$ 0	$ 0
Ending balance (in shares) at Dec. 31, 2015			19,619,124					0	0