Condensed Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit	Accumulated deficit Cumulative Effect, Period of Adoption, Adjustment
Beginning balance (in shares) at Dec. 31, 2018			54,738
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 6,564	$ 547	$ 3,386,958	$ (3,627)	$ (1,498,808)	$ 6,564
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)			131
Vesting of restricted stock units	0		$ 2	(2)
Exercise of stock options, shares (in shares)			189
Exercise of stock options	9,504		$ 2	9,502
Purchase of common stock under ESPP (in shares)			11
Purchase of common stock under ESPP	1,231			1,231
Stock-based compensation	32,341			32,341
Other comprehensive income (loss)	1,835				1,835
Net loss	(164,446)					(164,446)
Ending balance, shares (in shares) at Mar. 31, 2019			55,069
Ending balance at Mar. 31, 2019	$ 1,772,099		$ 551	3,430,030	(1,792)	(1,656,690)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Beginning balance (in shares) at Dec. 31, 2018			54,738
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 6,564	$ 547	3,386,958	(3,627)	(1,498,808)	$ 6,564
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss)	776
Net loss	(566,261)
Ending balance, shares (in shares) at Sep. 30, 2019			55,320
Ending balance at Sep. 30, 2019	1,471,710		$ 553	3,532,513	(2,851)	(2,058,505)
Beginning balance (in shares) at Mar. 31, 2019			55,069
Beginning balance at Mar. 31, 2019	1,772,099		$ 551	3,430,030	(1,792)	(1,656,690)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)			66
Vesting of restricted stock units	0		$ 1	(1)
Exercise of stock options, shares (in shares)			93
Exercise of stock options	3,973		$ 1	3,972
Stock-based compensation	55,111			55,111
Other comprehensive income (loss)	973				973
Net loss	(195,782)					(195,782)
Ending balance, shares (in shares) at Jun. 30, 2019			55,228
Ending balance at Jun. 30, 2019	1,636,374		$ 553	3,489,112	(819)	(1,852,472)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)			27
Exercise of stock options, shares (in shares)			51
Exercise of stock options	3,321		$ 1	3,320
Purchase of common stock under ESPP (in shares)			14
Purchase of common stock under ESPP	1,535			1,535
Stock-based compensation	38,546			38,546
Other comprehensive income (loss)	(2,032)				(2,032)
Net loss	(206,033)					(206,033)
Ending balance, shares (in shares) at Sep. 30, 2019			55,320
Ending balance at Sep. 30, 2019	$ 1,471,710		$ 553	3,532,513	(2,851)	(2,058,505)
Beginning balance (in shares) at Dec. 31, 2019	55,368		55,368
Beginning balance at Dec. 31, 2019	$ 1,284,993		$ 554	3,568,184	(1,893)	(2,281,852)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)			204
Vesting of restricted stock units	0		$ 2	(2)
Exercise of stock options, shares (in shares)			20
Exercise of stock options	750			750
Purchase of common stock under ESPP (in shares)			28
Purchase of common stock under ESPP	1,872			1,872
Stock-based compensation	36,335			36,335
Other comprehensive income (loss)	(906)				(906)
Net loss	(202,611)					(202,611)
Ending balance, shares (in shares) at Mar. 31, 2020			55,620
Ending balance at Mar. 31, 2020	$ 1,120,433		$ 556	3,607,139	(2,799)	(2,484,463)
Beginning balance (in shares) at Dec. 31, 2019	55,368		55,368
Beginning balance at Dec. 31, 2019	$ 1,284,993		$ 554	3,568,184	(1,893)	(2,281,852)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares (in shares)	55
Other comprehensive income (loss)	$ (2,330)
Net loss	$ (418,821)
Ending balance, shares (in shares) at Sep. 30, 2020	66,339		66,339
Ending balance at Sep. 30, 2020	$ 1,523,021		$ 663	4,227,254	(4,223)	(2,700,673)
Beginning balance (in shares) at Mar. 31, 2020			55,620
Beginning balance at Mar. 31, 2020	1,120,433		$ 556	3,607,139	(2,799)	(2,484,463)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon public offering, net of issuance costs (in shares)			10,455
Issuance of common stock upon public offering, net of issuance costs of $33,465	541,536		$ 105	541,431
Vesting of restricted stock units (in shares)			114
Vesting of restricted stock units	0		$ 1	(1)
Exercise of stock options, shares (in shares)			7
Exercise of stock options	347			347
Stock-based compensation	40,781			40,781
Other comprehensive income (loss)	399				399
Net loss	(21,465)					(21,465)
Ending balance, shares (in shares) at Jun. 30, 2020			66,196
Ending balance at Jun. 30, 2020	1,682,031		$ 662	4,189,697	(2,400)	(2,505,928)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)			62
Vesting of restricted stock units	0		$ 1	(1)
Exercise of stock options, shares (in shares)			28
Exercise of stock options	249			249
Purchase of common stock under ESPP (in shares)			53
Purchase of common stock under ESPP	1,902			1,902
Stock-based compensation	35,407			35,407
Other comprehensive income (loss)	(1,823)				(1,823)
Net loss	$ (194,745)					(194,745)
Ending balance, shares (in shares) at Sep. 30, 2020	66,339		66,339
Ending balance at Sep. 30, 2020	$ 1,523,021		$ 663	$ 4,227,254	$ (4,223)	$ (2,700,673)