Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Receivables from the Exercises of Options	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2014	$ 117,048	$ 3	$ 399,191	$ (835)	$ (95)	$ (281,216)
Beginning Balance (in shares) at Dec. 31, 2014		30,548,309
Stock-based compensation	94,934		94,934
Exercises of stock options	$ 28,228		27,393	835
Exercises of stock options (in shares)	757,038	757,038
Issuance of performance shares (in shares)		11,495
Issuance of common stock	$ 205,133		205,133
Issuance of common stock (in shares)		1,150,000
Unrealized gain on available-for-sale securities	(52)				(52)
Net loss	(239,284)					(239,284)
Ending Balance at Dec. 31, 2015	206,007	$ 3	726,651		(147)	(520,500)
Ending Balance (in shares) at Dec. 31, 2015		32,466,842
Stock-based compensation	117,264		117,264
Exercises of stock options	$ 576		576
Exercises of stock options (in shares)	43,751	46,668
Issuance of common stock	$ 161,854	$ 1	161,853
Issuance of common stock (in shares)		4,312,500
Unrealized gain on available-for-sale securities	134				134
Net loss	(276,011)					(276,011)
Ending Balance at Dec. 31, 2016	$ 209,824	$ 4	1,006,344		(13)	(796,511)
Ending Balance (in shares) at Dec. 31, 2016	36,826,010	36,826,010
Stock-based compensation	$ 108,735		108,735
Exercises of stock options (in shares)	557,080	557,080
Shares issued or RSUs vested under employee stock plans	$ 26,685		27,134	(449)
Shares issued or RSUs vested under employee stock plans (in shares)		768,841
Unrealized gain on available-for-sale securities	13				$ 13
Net loss	(291,955)					(291,955)
Ending Balance at Dec. 31, 2017	$ 53,302	$ 4	$ 1,142,213	$ (449)		$ (1,088,466)
Ending Balance (in shares) at Dec. 31, 2017	37,594,851	37,594,851