CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance (in shares) at Dec. 31, 2018		101,177,000
Beginning balance at Dec. 31, 2018	$ 1,301,965	$ 1,011	$ 4,175,139	$ (33,213)	$ (2,840,972)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings (in shares)		207,000
Exercise of common stock options, net of tax withholdings	11,409	$ 3	11,406
Issuance of common stock under equity plans (in shares)		4,000
Issuance of common stock under equity plans	(58)		(58)
Issuance of common stock under benefit plans (in shares)		12,000
Issuance of common stock under benefit plans	784		784
Issuance of common stock, net of offering costs (in shares)		5,000,000
Issuance of common stock, net of costs	381,900	$ 50	381,850
Stock-based compensation expense related to equity-classified awards	32,541		32,541
Other comprehensive income (loss), net of tax	360			360
Net loss	(181,915)				(181,915)
Balance (in shares) at Mar. 31, 2019		106,400,000
Ending balance at Mar. 31, 2019	1,546,986	$ 1,064	4,601,662	(32,853)	(3,022,887)
Balance (in shares) at Dec. 31, 2018		101,177,000
Beginning balance at Dec. 31, 2018	1,301,965	$ 1,011	4,175,139	(33,213)	(2,840,972)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss), net of tax	(1,158)
Net loss	(609,931)
Balance (in shares) at Sep. 30, 2019		111,325,000
Ending balance at Sep. 30, 2019	1,607,087	$ 1,113	5,091,248	(34,371)	(3,450,903)
Balance (in shares) at Mar. 31, 2019		106,400,000
Beginning balance at Mar. 31, 2019	1,546,986	$ 1,064	4,601,662	(32,853)	(3,022,887)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings (in shares)		203,000
Exercise of common stock options, net of tax withholdings	6,182	$ 2	6,180
Issuance of common stock under equity plans (in shares)		55,000
Issuance of common stock under equity plans	4,022		4,022
Issuance of common stock under benefit plans (in shares)		12,000
Issuance of common stock under benefit plans	1,089		1,089
Issuance of common stock, net of offering costs (in shares)		4,444,000
Issuance of common stock, net of costs	390,577	$ 44	390,533
Stock-based compensation expense related to equity-classified awards	30,798		30,798
Other comprehensive income (loss), net of tax	(2,978)			(2,978)
Net loss	(219,481)				(219,481)
Balance (in shares) at Jun. 30, 2019		111,114,000
Ending balance at Jun. 30, 2019	1,757,195	$ 1,110	5,034,284	(35,831)	(3,242,368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings (in shares)		191,000
Exercise of common stock options, net of tax withholdings	9,300	$ 2	9,298
Issuance of common stock under equity plans (in shares)		6,000
Issuance of common stock under equity plans	0
Issuance of common stock under benefit plans (in shares)		14,000
Issuance of common stock under benefit plans	1,002	$ 1	1,001
Stock-based compensation expense related to equity-classified awards	46,665		46,665
Other comprehensive income (loss), net of tax	1,460			1,460
Net loss	(208,535)				(208,535)
Balance (in shares) at Sep. 30, 2019		111,325,000
Ending balance at Sep. 30, 2019	$ 1,607,087	$ 1,113	5,091,248	(34,371)	(3,450,903)
Balance (in shares) at Dec. 31, 2019	112,188,000	112,188,000
Beginning balance at Dec. 31, 2019	$ 1,438,692	$ 1,122	5,201,176	(36,518)	(3,727,088)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings (in shares)		976,000
Exercise of common stock options, net of tax withholdings	54,221	$ 9	54,212
Issuance of common stock under equity plans (in shares)		4,000
Issuance of common stock under equity plans	0
Stock-based compensation expense related to equity-classified awards	34,578		34,578
Other comprehensive income (loss), net of tax	4,459			4,459
Net loss	(182,221)				(182,221)
Balance (in shares) at Mar. 31, 2020		113,168,000
Ending balance at Mar. 31, 2020	$ 1,349,729	$ 1,131	5,289,966	(32,059)	(3,909,309)
Balance (in shares) at Dec. 31, 2019	112,188,000	112,188,000
Beginning balance at Dec. 31, 2019	$ 1,438,692	$ 1,122	5,201,176	(36,518)	(3,727,088)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss), net of tax	(675)
Net loss	$ (614,741)
Balance (in shares) at Sep. 30, 2020	116,143,000	116,143,000
Ending balance at Sep. 30, 2020	$ 1,215,023	$ 1,161	5,592,884	(37,193)	(4,341,829)
Balance (in shares) at Mar. 31, 2020		113,168,000
Beginning balance at Mar. 31, 2020	1,349,729	$ 1,131	5,289,966	(32,059)	(3,909,309)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings (in shares)		1,233,000
Exercise of common stock options, net of tax withholdings	91,873	$ 12	91,861
Issuance of common stock under equity plans (in shares)		283,000
Issuance of common stock under equity plans	5,301	$ 3	5,298
Issuance of common stock, net of offering costs (in shares)		963,000
Issuance of common stock, net of costs	99,498	$ 10	99,488
Stock-based compensation expense related to equity-classified awards	33,707		33,707
Other comprehensive income (loss), net of tax	(1,153)			(1,153)
Net loss	(179,229)				(179,229)
Balance (in shares) at Jun. 30, 2020		115,647,000
Ending balance at Jun. 30, 2020	1,399,726	$ 1,156	5,520,320	(33,212)	(4,088,538)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, net of tax withholdings (in shares)		494,000
Exercise of common stock options, net of tax withholdings	32,212	$ 5	32,207
Issuance of common stock under equity plans (in shares)		2,000
Issuance of common stock under equity plans	129	$ 0	129
Stock-based compensation expense related to equity-classified awards	40,228		40,228
Other comprehensive income (loss), net of tax	(3,981)			(3,981)
Net loss	$ (253,291)				(253,291)
Balance (in shares) at Sep. 30, 2020	116,143,000	116,143,000
Ending balance at Sep. 30, 2020	$ 1,215,023	$ 1,161	$ 5,592,884	$ (37,193)	$ (4,341,829)