Consolidated Statements of Stockholders' Equity (Deficit) Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Beginning balance (in shares) at Dec. 31, 2015		214,781,573
Beginning balance at Dec. 31, 2015	$ 981,451	$ 21	$ 737,820	$ 691	$ 242,919
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		10,881,704
Issuance of common stock upon exercise of stock options	25,814	$ 2	25,812
Stock-based compensation expense	79,107		79,107
Taxes related to net share settlement of restricted stock units	(4,939)		(4,939)
Cumulative effect adjustment related to recognition of previously unrecognized excess tax benefits from adoption of ASU 2016-09	21,545		21,545
Net loss	(102,777)				(102,777)
Other comprehensive Income (loss)	(1,669)			(1,669)
Ending balance (in shares) at Dec. 31, 2016		225,663,277
Ending balance at Dec. 31, 2016	998,532	$ 23	859,345	(978)	140,142
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		13,093,245
Issuance of common stock upon exercise of stock options	19,011	$ 1	19,010
Stock-based compensation expense	92,081		92,081
Taxes related to net share settlement of restricted stock units	(14,376)		(14,376)
Cumulative effect adjustment related to recognition of previously unrecognized excess tax benefits from adoption of ASU 2016-09 | Accounting Standards Update 2016-09 [Member]	4,938				4,938
Net loss	(277,192)				(277,192)
Other comprehensive Income (loss)	969			969
Ending balance (in shares) at Dec. 31, 2017		238,756,522
Ending balance at Dec. 31, 2017	823,963	$ 24	956,060	(9)	(132,112)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		13,606,319
Issuance of common stock upon exercise of stock options	21,470	$ 1	21,469
Stock-based compensation expense	96,953		96,953
Taxes related to net share settlement of restricted stock units	(19,436)		(19,436)
Net loss	(185,829)				(185,829)
Other comprehensive Income (loss)	(57)			(57)
Ending balance (in shares) at Dec. 31, 2018		252,362,841
Ending balance at Dec. 31, 2018	$ 735,938	$ 25	$ 1,055,046	$ (66)	$ (319,067)