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 Loss [Member]	Accumulated Deficit [Member]	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]
Balances at Dec. 31, 2014						$ 3,963	$ 8,630			$ 12,593
Balances (Shares) at Dec. 31, 2014						2,393,000	1,906,000
Balances (Shares) at Dec. 31, 2014		2,645,000
Balances at Dec. 31, 2014	$ (18,291)		$ 10,518		$ (28,809)
Issuance of stock, net of transaction costs	145,184	$ 1	145,183					$ 26,021		26,021
Issuance of stock, net of transaction costs, shares		7,245,000						4,047,000	7,367,000
Issuance of convertible preferred stock for the conversion of outstanding related party debt								$ 3,792		3,792
Issuance of convertible preferred stock for the conversion of outstanding related party debt, Shares								585,000
Issuance of convertible preferred stock, net of transaction costs									$ 67,998	67,998
Loss (gain) on extinguishment of convertible preferred stock	759		759			$ 1,317	$ (2,076)			(759)
Accretion (decretion) of convertible preferred stock	(1,747)		(1,747)			(2,280)	1,338	$ 187	2,502	1,747
Conversion of convertible preferred stock into common stock upon initial public offering	111,392	$ 2	111,390			$ (3,000)	$ (7,892)	$ (30,000)	$ (70,500)	$ (111,392)
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		16,298,000				(2,393,000)	(1,906,000)	(4,632,000)	(7,367,000)
Discount on related party promissory notes	13		13
Exercise of stock options	107		107
Exercise of stock options, Shares		125,000
Stock-based compensation expense	2,921		2,921
Unrealized loss on available-for-sale securities	(719)			$ (719)
Net loss	(22,811)				(22,811)
Balances (Shares) at Dec. 31, 2015		26,313,000
Balances at Dec. 31, 2015	216,808	$ 3	269,144	(719)	(51,620)
Exercise of stock options	179		179
Exercise of stock options, Shares		163,000
Stock-based compensation expense	7,031		7,031
Unrealized loss on available-for-sale securities	686			686
Net loss	$ (62,967)				(62,967)
Balances (Shares) at Dec. 31, 2016	0
Balances (Shares) at Dec. 31, 2016		26,477,000
Balances at Dec. 31, 2016	$ 161,737	$ 3	276,354	(33)	(114,587)
Issuance of stock, net of transaction costs	81,489		81,489
Issuance of stock, net of transaction costs, shares		4,255,000
Exercise of stock options	2,493		2,493
Exercise of stock options, Shares		516,000
Issuance of common stock under employee stock purchase plan	556		556
Issuance of common stock under employee stock purchase plan, shares		48,000
Stock-based compensation expense	10,605		10,605
Unrealized loss on available-for-sale securities	(682)			(682)
Net loss	$ (73,169)				(73,169)
Balances (Shares) at Dec. 31, 2017	0
Balances (Shares) at Dec. 31, 2017		31,295,000
Balances at Dec. 31, 2017	$ 183,029	$ 3	$ 371,497	$ (715)	$ (187,756)