Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Deficit - 9 months ended Sep. 30, 2017 - USD ($) $ in Thousands	Total	Common Stock	Additional Paid -in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock, Series C2	Redeemable Convertible Preferred Stock, Series D	Redeemable Convertible Preferred Stock, Series C, C1, D	Redeemable Convertible Preferred Stock, Series C, C1, D Additional Paid -in Capital	Redeemable Convertible Preferred Stock, Series C, C1, C2, and D	Redeemable Convertible Preferred Stock, Series C, C1, C2, and D Additional Paid -in Capital	Redeemable Convertible Preferred Stock, Series C, C1, C2, and D Accumulated Deficit
Shares outstanding (in shares) at Dec. 31, 2016						6,875,514	0	0
Beginning balance at Dec. 31, 2016						$ 168,173
Shares outstanding (in shares) at Dec. 31, 2016		4,067,717
Beginning balance at Dec. 31, 2016	$ (151,197)	$ 4	$ 0	$ (151,102)	$ (99)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of convertible debt to Series C-2 preferred stock (in shares)							687,474
Conversion of convertible debt to Series C-2 preferred stock							$ 19,527
Sale of Series D preferred stock, net of issuance costs (in shares)								1,117,578
Sale of Series D preferred stock, net of issuance costs								$ 36,494
Stock compensation expense	1,800		1,800
Accretion of preferred stock									$ 1,093		$ 11,255
Accretion of preferred stock									$ (1,093)	$ (1,093)	$ (11,255)	$ (707)	$ (10,548)
Foreign currency translation adjustment	(10)				(10)
Net loss	(29,351)			(29,351)
Shares outstanding (in shares) at Sep. 30, 2017						8,680,566	687,474	1,369,863
Ending balance at Sep. 30, 2017						$ 236,542
Shares outstanding (in shares) at Sep. 30, 2017		4,067,717
Ending balance at Sep. 30, 2017	$ (191,106)	$ 4	$ 0	$ (191,001)	$ (109)