Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	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
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	346,478
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)
Balance (in shares) at Dec. 31, 2013	15,357,413
Balance at Dec. 31, 2012		610	$ (41,975)		$ 23,975		(41,365)
Balance (in shares) at Dec. 31, 2012	346,478
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
Preferred shares converted into common stock	$ 9	65,216			$ (57,478)	$ (7,750)	65,225
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
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	346,478
Common Stock, Shares, Issued							396,414
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 11, 2013							15,357,413
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 11, 2013							15,357,413
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued							396,414
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 01, 2013							14,607,413
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 01, 2013							14,607,413
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued							396,414
Common Stock, Shares, Issued							14,607,413
Common Stock, Shares, Issued							15,357,413
Common Stock, Shares, Issued	15,357,413
Balance (in shares) at Dec. 31, 2013	15,357,413
Balance (in shares) at Dec. 31, 2014	20,352,876						20,352,876
Balance (in shares) at Dec. 31, 2013	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
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
Balance (in shares) at Dec. 31, 2014	20,352,876						20,352,876
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	15,357,413						20,352,876
Common Stock, Shares, Issued	20,352,876						20,352,876
Balance (in shares) at Dec. 31, 2015	22,518,907						22,518,907
Balance (in shares) at Dec. 31, 2014	20,352,876						20,352,876
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		26					$ 26
Issuance of common stock from public offering, net of issuance costs	$ 3	189,980					189,983
Issuance of common stock from public offering, net of issuance costs (in shares)	2,012,500
Exercise of stock options		1,177					$ 1,177
Exercise of stock options (in shares)	128,086						128,086
Exercise of warrants (in shares)	25,445
Stock-based compensation		12,726					$ 12,726
Other comprehensive loss				(423)			(423)
Net loss			(49,784)				(49,784)
Balance at Dec. 31, 2015	$ 23	$ 441,940	$ (154,222)	$ (482)			$ 287,259
Balance (in shares) at Dec. 31, 2015	22,518,907						22,518,907
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	20,352,876						20,352,876
Common Stock, Shares, Issued	22,518,907						22,518,907