Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2014		37,100,513
Beginning balance at Dec. 31, 2014	$ 424,366	$ 37	$ 591,334	$ (57)	$ (166,948)
Unrealized (loss) gain on available-for-sale securities	(261)			(261)	0
Net loss	(117,732)				(117,732)
Stock-based compensation expense	31,574		31,574
Vesting of restricted stock (in shares)		8,522
Vesting of restricted stock	6		6
Issuance of common stock upon exercise of stock options and employee stock purchase plan (in shares)		587,467
Issuance of common stock under stock incentive and employee stock purchase plans	7,136	$ 1	7,135
Issuance of common stock for follow-on offering	29		29
Ending balance (in shares) at Dec. 31, 2015		37,696,502
Ending balance at Dec. 31, 2015	345,118	$ 38	630,078	(318)	(284,680)
Unrealized (loss) gain on available-for-sale securities	5			5	0
Net loss	(198,471)				(198,471)
Stock-based compensation expense	42,086		42,086
Issuance of common stock upon exercise of stock options and employee stock purchase plan (in shares)		647,539
Issuance of common stock under stock incentive and employee stock purchase plans	7,703	$ 0	7,703
Issuance of common stock for follow-on offering (in shares)		3,876,403
Issuance of common stock for follow-on offering	162,150	$ 4	162,146
Ending balance (in shares) at Dec. 31, 2016		42,220,444
Ending balance at Dec. 31, 2016	358,591	$ 42	842,013	(313)	(483,151)
Unrealized (loss) gain on available-for-sale securities	(1,076)			(1,076)	0
Net loss	(314,670)				(314,670)
Stock-based compensation expense	$ 47,809		47,809
Vesting of restricted stock (in shares)	7,500
Issuance of common stock upon exercise of stock options and employee stock purchase plan (in shares)	730,478	797,629
Issuance of common stock under stock incentive and employee stock purchase plans	$ 14,190	$ 1	14,189
Issuance of common stock for follow-on offering (in shares)		5,808,080
Issuance of common stock for follow-on offering	270,250	$ 6	270,244
Other	409		649		(240)
Ending balance (in shares) at Dec. 31, 2017		48,826,153
Ending balance at Dec. 31, 2017	$ 375,503	$ 49	$ 1,174,904	$ (1,389)	$ (798,061)