Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]	Series D Convertible Preferred Stock [Member]
Stockholders' (deficit), value, beginning balance at Dec. 31, 2014	$ (26,721)	$ 7	$ 1,104		$ (27,832)
Convertible preferred stock, shares, beginning balance at Dec. 31, 2014						22,866,987
Convertible preferred stock, value, beginning balance at Dec. 31, 2014						$ 136,077
Stockholders' (deficit), shares, beginning balance at Dec. 31, 2014		6,890,250
Issuance of common stock upon exercise of stock options, value	93		93
Issuance of common stock upon exercise of stock options, shares		232,970
Issuance of common stock upon exercise of common stock warrant, value	169	$ 1	168
Issuance of common stock upon exercise of common stock warrant, shares		546,672
Issuance of common stock upon completion of initial public offering, net of offering costs, value	139,267	$ 8	139,259
Issuance of common stock upon completion of initial public offering, net of offering costs, shares		8,545,138
Stock-based compensation expense	9,694		9,694
Issuance costs of convertible preferred stock							$ (24)
Reclassification of preferred stock warrant liability	1,589		1,589
Conversion of convertible preferred stock upon listing, value	136,053	$ 23	136,030			$ (136,053)
Conversion of convertible preferred stock upon listing, shares		22,866,987				(22,866,987)
Unrealized gain (loss) on investments	30			$ 30
Net loss	(54,780)				(54,780)
Stockholders' (deficit), value, ending balance at Dec. 31, 2015	205,394	$ 39	287,937	30	(82,612)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2015		39,082,017
Issuance of common stock upon exercise of stock options, value	2,138	$ 1	2,137
Issuance of common stock upon exercise of stock options, shares		1,273,736
Stock-based compensation expense	16,857		16,857
Unrealized gain (loss) on investments	(179)			(179)
Net loss	(91,579)				(91,579)
Stockholders' (deficit), value, ending balance at Dec. 31, 2016	$ 132,631	$ 40	306,931	(149)	(174,191)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2016	40,355,753	40,355,753
Issuance of common stock upon exercise of stock options, value	$ 116		116
Issuance of common stock upon exercise of stock options, shares	174,386	174,386
Issuance of common stock upon vesting of RSUs	$ (33)		(33)
Issuance of common stock upon vesting of RSUs, shares		40,876
Stock-based compensation expense	17,362		17,362
Unrealized gain (loss) on investments	3			3
Net loss	(89,380)				(89,380)
Stockholders' (deficit), value, ending balance at Dec. 31, 2017	$ 60,699	$ 40	$ 324,376	$ (146)	$ (263,571)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2017	40,571,015	40,571,015