Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Class A and B Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balances at Dec. 31, 2013	$ (86,147)		$ 1	$ 8,731	$ (94,892)	$ 13
Balances, Shares at Dec. 31, 2013			11,668,000
Balances, Convertible Preferred Stock at Dec. 31, 2013		$ 133,809
Balances, Convertible Preferred Stock, Shares at Dec. 31, 2013		18,240,000
Stock option exercises	1,437			1,437
Stock option exercises, Shares			642,000
Stock-based compensation	5,766			5,766
Comprehensive loss	(32,883)				(32,872)	(11)
Balances at Dec. 31, 2014	(111,827)		$ 1	15,934	(127,764)	2
Balances, Shares at Dec. 31, 2014			12,310,000
Balances, Convertible Preferred Stock at Dec. 31, 2014		$ 133,809
Balances, Convertible Preferred Stock, Shares at Dec. 31, 2014		18,240,000
Stock option exercises	2,670			2,670
Stock option exercises, Shares			587,000
Stock-based compensation	7,815			7,815
Issuance of common stock warrants	90			90
Comprehensive loss	(41,009)				(41,007)	(2)
Balances at Dec. 31, 2015	$ (142,261)		$ 1	26,509	(168,771)
Balances, Shares at Dec. 31, 2015	12,897,001		12,897,000
Balances, Convertible Preferred Stock at Dec. 31, 2015	$ 133,809	$ 133,809
Balances, Convertible Preferred Stock, Shares at Dec. 31, 2015		18,240,000
Proceeds from initial public offering, net of underwriters' discounts	102,672		$ 1	102,671
Proceeds from initial public offering, net of underwriters' discounts, Shares			6,900,000
Initial public offering costs	(3,698)			(3,698)
Conversion of preferred stock to Class B common stock	133,809	$ (133,809)	$ 2	133,807
Conversion of preferred stock to Class B common stock, Shares		(18,240,000)	18,240,000
Exercise of warrants	616			616
Exercise of warrants, Shares			40,000
Stock option exercises	1,333			1,333
Stock option exercises, Shares			261,000
Stock-based compensation	10,459			10,459
Issuance of common stock warrants	285			285
Comprehensive loss	(31,647)				(31,553)	(94)
Balances at Dec. 31, 2016	$ 71,568		$ 4	$ 271,982	$ (200,324)	$ (94)
Balances, Shares at Dec. 31, 2016			38,338,000