Condensed Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2019		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
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]
Other comprehensive income (loss)	(507)
Net loss	(224,076)
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)
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	0	0
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)
Beginning balance (in shares) at Dec. 31, 2020	66,432	66,432
Beginning balance at Dec. 31, 2020	$ 1,355,056	$ 665	4,260,443	(5,505)	(2,900,547)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		294
Vesting of restricted stock units	0	$ 3	(3)
Exercise of stock options, shares (in shares)		207
Exercise of stock options	1,219	$ 2	1,217
Purchase of common stock under ESPP (in shares)		67
Purchase of common stock under ESPP	1,707	$ 1	1,706
Stock-based compensation	36,090		36,090
Issuance of stock to settle liability-classified restricted stock units (in shares)		422
Issuance of unrestricted stock awards to settle accrued employee compensation	12,013	$ 4	12,009
Other comprehensive income (loss)	56			56
Net loss	(205,808)				(205,808)
Ending balance, shares (in shares) at Mar. 31, 2021		67,422
Ending balance at Mar. 31, 2021	$ 1,200,333	$ 675	4,311,462	(5,449)	(3,106,355)
Beginning balance (in shares) at Dec. 31, 2020	66,432	66,432
Beginning balance at Dec. 31, 2020	$ 1,355,056	$ 665	4,260,443	(5,505)	(2,900,547)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares (in shares)	209
Other comprehensive income (loss)	$ (272)
Net loss	$ (447,510)
Ending balance, shares (in shares) at Jun. 30, 2021	67,551	67,551
Ending balance at Jun. 30, 2021	$ 984,561	$ 676	4,337,719	(5,777)	(3,348,057)
Beginning balance (in shares) at Mar. 31, 2021		67,422
Beginning balance at Mar. 31, 2021	1,200,333	$ 675	4,311,462	(5,449)	(3,106,355)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		127
Vesting of restricted stock units	0	$ 1	(1)
Exercise of stock options, shares (in shares)		2
Exercise of stock options	36		36
Stock-based compensation	26,222		26,222
Other comprehensive income (loss)	(328)			(328)
Net loss	$ (241,702)				(241,702)
Ending balance, shares (in shares) at Jun. 30, 2021	67,551	67,551
Ending balance at Jun. 30, 2021	$ 984,561	$ 676	$ 4,337,719	$ (5,777)	$ (3,348,057)