Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock.	Additional Paid-In Capital	Deficit Accumulated During the Development Stage	Accumulated Other Comprehensive Loss	Series A preferred stock	Series A-1 preferred stock	Total
Balance at Dec. 31, 2011		$ 201	$ (30,233)				$ (30,032)
Balance at Dec. 31, 2011					23,975
Balance (in shares) at Dec. 31, 2011	307,742
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		41					41
Exercise of stock options (in shares)	38,736
Beneficial conversion feature from issuance of convertible notes		288					288
Stock-based compensation		80					80
Net loss			(11,742)				(11,742)
Balance at Dec. 31, 2012		610	(41,975)		23,975		(41,365)
Balance (in shares) at Dec. 31, 2012	346,478
Increase (Decrease) in Temporary Equity
Issuance of preferred stock in exchange for convertible promissory notes, net of issuance costs					16,623	7,750
Issuance of preferred stock in exchange for convertible promissory notes, net of issuance costs (in shares)					16,623,092	6,750,000
Issuance of Series A preferred stock, net of issuance costs					16,880
Issuance of Series A preferred stock, net of issuance costs (in shares)					17,000,000
Preferred shares converted into common stock	9	65,216			(57,478)	(7,750)	65,225
Preferred shares converted into common stock (in shares)	9,210,999				(57,598,092)	(6,750,000)
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		21					21
Early exercise of stock options and vesting of restricted stock (in shares)	25,765
Issuance of common stock from public offering, net of issuance costs	6	72,188					72,194
Issuance of common stock from public offering, net of issuance costs (in shares)	5,750,000
Reclassification of warrants from liabilities to equity		2,852					2,852
Exercise of stock options		28					28
Exercise of stock options (in shares)	24,171
Stock-based compensation		1,227					1,227
Other comprehensive loss				(3)			(3)
Net loss			(26,088)				(26,088)
Balance at Dec. 31, 2013	15	142,142	(68,063)	(3)			74,091
Balance (in shares) at Dec. 31, 2013	15,357,413						15,357,413
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		39					39
Issuance of common stock from public offering, net of issuance costs	5	91,620					91,625
Issuance of common stock from public offering, net of issuance costs (in shares)	4,887,500
Issued warrants in connection with debt issuance		78					78
Exercise of stock options		473					473
Exercise of stock options (in shares)	107,963						107,963
Stock-based compensation		3,679					3,679
Other comprehensive loss				(56)			(56)
Net loss			(36,375)				(36,375)
Balance at Dec. 31, 2014	$ 20	$ 238,031	$ (104,438)	$ (59)			$ 133,554