CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ 1,766,431	$ 997	$ 3,947,552	$ (34,433)	$ (2,147,685)
Balance (in shares) at Dec. 31, 2017		99,667
Cumulative effect adjustment from the adoption of new revenue standard	68,210				68,210
Exercise of common stock options, net of tax withholdings	41,890	$ 8	41,882
Exercise of common stock options, net of tax withholdings (in shares)		795
Issuance of common stock under other types of equity plans	567		567
Issuance of common stock under other types of equity plans (in shares)		4
Issuance of common stock under equity plans, net of tax withholdings	(122)		(122)
Issuance of common stock under equity plans, net of tax withholdings (in shares)		2
Stock-based compensation expense related to equity-classified awards	19,463		19,463
Other comprehensive gain (loss), net of tax	(420)			(420)
Net loss	(141,214)				(141,214)
Balance at Mar. 31, 2018	1,754,805	$ 1,005	4,009,342	(34,853)	(2,220,689)
Balance (in shares) at Mar. 31, 2018		100,468
Balance at Dec. 31, 2018	$ 1,301,965	$ 1,011	4,175,139	(33,213)	(2,840,972)
Balance (in shares) at Dec. 31, 2018	101,177	101,177
Exercise of common stock options, net of tax withholdings	$ 11,409	$ 3	11,406
Exercise of common stock options, net of tax withholdings (in shares)		207
Issuance of common stock under other types of equity plans	784		784
Issuance of common stock under other types of equity plans (in shares)		11
Issuance of common stock under equity plans, net of tax withholdings	(58)		(58)
Issuance of common stock under equity plans, net of tax withholdings (in shares)		5
Issuance of common stock, net of offering costs	381,900	$ 50	381,850
Issuance of common stock, net of offering costs (in shares)		5,000
Stock-based compensation expense related to equity-classified awards	32,541		32,541
Other comprehensive gain (loss), net of tax	360			360
Net loss	(181,915)				(181,915)
Balance at Mar. 31, 2019	$ 1,546,986	$ 1,064	$ 4,601,662	$ (32,853)	$ (3,022,887)
Balance (in shares) at Mar. 31, 2019	106,400	106,400