Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional Paid-in Capital	Notes Receivable from Officers	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Balance at Dec. 31, 2014	$ 1	$ 8,664	$ (192)		$ (179,808)	$ (171,335)
Balance (in shares) at Dec. 31, 2014	6,879
Issuance of common stock upon exercise of stock options		1,258				1,258
Issuance of common stock upon exercise of stock options (in shares)	1,170
Stock based compensation		7,326				7,326
Officer loan receivable repayment			$ 192			192
Issuance of common stock in connection with the initial public offering	$ 1	182,465				182,466
Issuance of common stock in connection with the initial public offering (in shares)	10,900
Issuance costs of Initial Public Offering		(4,036)				(4,036)
Conversion of all Preferred Stock into Common Stock upon the completion of the initial public offering	$ 3	240,582				240,585
Conversion of all Preferred Stock into Common Stock upon the completion of the initial public offering (in shares)	31,397
Unrealized (loss) gain on available-for-sale investments				$ (1,416)		(1,416)
Net loss					(70,275)	(70,275)
Balance at Dec. 31, 2015	$ 5	436,259		(1,416)	(250,083)	184,765
Balance (in shares) at Dec. 31, 2015	50,346
Issuance of common stock upon exercise of stock options		3,595				3,595
Issuance of common stock upon exercise of stock options (in shares)	1,913
Issuance of common stock under employee stock purchase plan		2,589				2,589
Issuance of common stock under employee stock purchase plan (in shares)	341
Vesting of restricted stock (in shares)	65
Stock based compensation		10,601				10,601
Unrealized (loss) gain on available-for-sale investments				691		691
Net loss					(95,765)	(95,765)
Balance at Dec. 31, 2016	$ 5	453,044		(725)	(345,848)	$ 106,476
Balance (in shares) at Dec. 31, 2016	52,665					52,665
Issuance of common stock upon exercise of stock options	$ 1	2,314				$ 2,315
Issuance of common stock upon exercise of stock options (in shares)	565
Issuance of common stock under employee stock purchase plan		3,344				3,344
Issuance of common stock under employee stock purchase plan (in shares)	446
Issuance of common stock in exchange for commitment to provide debt financing		2,448				2,448
Issuance of common stock in exchange for commitment to provide debt financing (in shares)	300
Vesting of restricted stock (in shares)	64
Stock based compensation		11,402				11,402
Unrealized (loss) gain on available-for-sale investments				(41)		(41)
Net loss					(136,314)	(136,314)
Balance at Dec. 31, 2017	$ 6	$ 472,552		$ (766)	$ (482,162)	$ (10,370)
Balance (in shares) at Dec. 31, 2017	54,040					54,040