Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ 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, 2014	$ (111,827)		$ 1	$ 15,934	$ (127,764)	$ 2
Balances, Convertible Preferred Stock, Shares at Dec. 31, 2014		18,240
Balances, Convertible Preferred Stock at Dec. 31, 2014		$ 133,809
Balances, Shares at Dec. 31, 2014			12,310
Stock option exercises	2,670			2,670
Stock option exercises, Shares			587
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, Convertible Preferred Stock, Shares at Dec. 31, 2015		18,240
Balances at Dec. 31, 2015		$ 133,809
Balances, Shares at Dec. 31, 2015			12,897
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
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)	18,240
Exercise of warrants	616			616
Exercise of warrants, Shares			40
Stock option exercises	1,333			1,333
Stock option exercises, Shares			261
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
Stock option exercises	21,868			21,868
Stock option exercises, Shares			3,335
Vesting of restricted stock units, Shares			128
Issuance of stock under employee stock purchase plan	4,381			4,381
Issuance of stock under employee stock , Shares			312
Stock-based compensation	16,070			16,070
Comprehensive loss	(25,637)				(25,621)	(16)
Balances at Dec. 31, 2017	$ 88,250		$ 4	$ 314,301	$ (225,945)	$ (110)
Balances, Shares at Dec. 31, 2017			42,113