Condensed Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Initial Public Offering [Member]	Follow On Public Offering [Member]	Common Stock [Member]	Common Stock [Member] Initial Public Offering [Member]	Common Stock [Member] Follow On Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Initial Public Offering [Member]	Additional Paid-in Capital [Member] Follow On Public Offering [Member]	Accumulated Deficit [Member]
Balance, Value at Dec. 31, 2013		$ 65,586
Balance, Value at Dec. 31, 2013	$ (54,075)				$ 4			$ 8,377			$ (62,456)
Balance, Shares at Dec. 31, 2013		29,186,929
Balance, Shares at Dec. 31, 2013					4,288,243
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value			$ 56,247			$ 6			$ 56,241
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares						5,750,000
Preferred stock converted to common shares, Value	65,586	$ (65,586)			$ 13			65,573
Preferred stock converted to common shares, Shares		29,186,929			12,554,171
Stock-based compensation expense	1,031							1,031
Shares issued upon exercise of stock options, Value	119							119
Shares issued upon exercise of stock options, Shares					179,800
Shares issued upon cashless exercise of warrants, Shares					6,383
Net loss	(13,573)										(13,573)
Balance, Value at Sep. 30, 2014	55,335				$ 23			131,341			(76,029)
Balance, Shares at Sep. 30, 2014					22,778,597
Balance, Value at Dec. 31, 2014	51,662				$ 23			131,840			(80,201)
Balance, Shares at Dec. 31, 2014					22,802,039
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value				$ 75,231			$ 4			$ 75,227
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares							4,327,956
Stock-based compensation expense	1,807							1,807
Shares issued upon exercise of stock options, Value	323							323
Shares issued upon exercise of stock options, Shares					101,588
Net loss	(15,160)										(15,160)
Balance, Value at Sep. 30, 2015	$ 113,863				$ 27			$ 209,197			$ (95,361)
Balance, Shares at Sep. 30, 2015					27,231,583