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, 2018		54,738
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 547	$ 3,386,958	$ (3,627)	$ (1,498,808)
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 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)
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)
Vesting of restricted stock units (in shares)		204
Vesting of restricted stock units	$ 0	$ 2	(2)
Exercise of stock options, shares (in shares)	20	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 loss	(906)			(906)
Net loss	$ (202,611)				(202,611)
Ending balance, shares (in shares) at Mar. 31, 2020	55,620	55,620
Ending balance at Mar. 31, 2020	$ 1,120,433	$ 556	$ 3,607,139	$ (2,799)	$ (2,484,463)