CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning 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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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 equity plans	(58)		(58)
Issuance of common stock under equity plans (in shares)		4
Issuance of common stock under benefit plans	784		784
Issuance of common stock under benefit plans (in shares)		12
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 income	360			360
Net loss	(181,915)				(181,915)
Ending 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
Beginning balance at Dec. 31, 2019	$ 1,438,692	$ 1,122	5,201,176	(36,518)	(3,727,088)
Balance (in shares) at Dec. 31, 2019	112,188	112,188
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings	$ 54,221	$ 9	54,212
Exercise of common stock options, net of tax withholdings (in shares)		976
Issuance of common stock under equity plans	0		0
Issuance of common stock under equity plans (in shares)		4
Stock-based compensation expense related to equity-classified awards	34,578		34,578
Other comprehensive income	4,459			4,459
Net loss	(182,221)				(182,221)
Ending balance at Mar. 31, 2020	$ 1,349,729	$ 1,131	$ 5,289,966	$ (32,059)	$ (3,909,309)
Balance (in shares) at Mar. 31, 2020	113,168	113,168