Statements of Convertible Preferred Stock and Changes in Stockholders' Equity (Deficit), Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common stock Redeemable Convertible Preferred Stock	Common stock Convertible notes Convertible Promissory Notes, April 2014	Common stock Private Placement	Common stock Initial public offering	Common stock	Additional Paid-in Capital Redeemable Convertible Preferred Stock	Additional Paid-in Capital Convertible notes Convertible Promissory Notes, April 2014	Additional Paid-in Capital Private Placement	Additional Paid-in Capital Initial public offering	Additional Paid-in Capital	Deficit Accumulated during the Development Stage	Redeemable Convertible Preferred Stock	Convertible notes Convertible Promissory Notes, April 2014	Private Placement	Initial public offering	Total
Balance at Dec. 31, 2012										$ 45,385	$ (103,994)					$ (58,609)
Balance (in shares) at Dec. 31, 2012					45,303
Increase (decrease) in stockholders' equity (deficit)
Share based compensation-stock options										1,338						1,338
Issuance of common stock for employee bonuses										80						80
Issuance of common stock for employee bonuses (in shares)					11,981
Exercise of stock options										70						70
Exercise of stock options (in shares)					52,037
Net loss											(14,321)					(14,321)
Balance at Dec. 31, 2013										46,873	(118,315)					(71,442)
Balance (in shares) at Dec. 31, 2013					109,321
Increase (decrease) in stockholders' equity (deficit)
Share based compensation-stock options										1,381						1,381
Issuance of common stock for employee bonuses										80						80
Issuance of common stock for employee bonuses (in shares)					9,983
Conversion to common stock	$ 1					$ 69,232	$ 3,020					$ 69,233	$ 3,020
Conversion to common stock (in shares)	8,803,547	503,450
Common stock issued, net of expenses				$ 1					$ 49,743						$ 49,744
Common stock issued (in shares)				9,166,667
Exercise of stock options										67						67
Exercise of stock options (in shares)					41,904
Net loss											(16,077)					(16,077)
Balance at Dec. 31, 2014					$ 2					170,396	(134,392)					36,006
Balance (in shares) at Dec. 31, 2014					18,634,872
Increase (decrease) in stockholders' equity (deficit)
Share based compensation-stock options										2,965						2,965
Common stock issued, net of expenses								$ 19,330						$ 19,330
Common stock issued (in shares)			3,418,804
Fair value of common stock warrant issued with debt financing										1,184						1,184
Exercise of stock options										593						593
Exercise of stock options (in shares)					261,936
Net loss											(30,335)					(30,335)
Balance at Dec. 31, 2015					$ 2					$ 194,468	$ (164,727)					$ 29,743
Balance (in shares) at Dec. 31, 2015					22,315,612