Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ 1,623,432	$ 494	$ 2,540,951	$ (4,205)	$ (913,808)
Beginning balance, shares at Dec. 31, 2017		49,406
Adjustment to beginning accumulated deficit from adoption of accounting standard update | ASU 2014-09 [Member]	(29,375)				(29,375)
Vesting of restricted stock units		$ 1	(1)
Vesting of restricted stock units, shares		74
Issuance of common stock upon public offering, net of issuance costs	48,701	$ 3	48,698
Issuance of common stock upon public offering, net of issuance costs, shares		277
Exercise of stock options	19,730	$ 3	19,727
Exercise of stock options, shares		301
Purchase of common stock under ESPP	687		687
Purchase of common stock under ESPP, shares		9
Stock-based compensation	22,995		22,995
Other comprehensive income (loss)	(844)			(844)
Net loss	(115,126)				(115,126)
Ending balance at Mar. 31, 2018	1,570,201	$ 501	2,633,057	(5,049)	(1,058,308)
Ending balance, shares at Mar. 31, 2018		50,067
Beginning balance at Dec. 31, 2017	1,623,432	$ 494	2,540,951	(4,205)	(913,808)
Beginning balance, shares at Dec. 31, 2017		49,406
Other comprehensive income (loss)	(1,189)
Net loss	(261,120)
Ending balance at Jun. 30, 2018	1,457,534	$ 502	2,666,729	(5,394)	(1,204,303)
Ending balance, shares at Jun. 30, 2018		50,225
Beginning balance at Mar. 31, 2018	1,570,201	$ 501	2,633,057	(5,049)	(1,058,308)
Beginning balance, shares at Mar. 31, 2018		50,067
Vesting of restricted stock units, shares		33
Exercise of stock options	5,617	$ 1	5,616
Exercise of stock options, shares		125
Stock-based compensation	28,056		28,056
Other comprehensive income (loss)	(345)			(345)
Net loss	(145,996)				(145,996)
Ending balance at Jun. 30, 2018	1,457,534	$ 502	2,666,729	(5,394)	(1,204,303)
Ending balance, shares at Jun. 30, 2018		50,225
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 547	3,386,958	(3,627)	(1,498,808)
Beginning balance, shares at Dec. 31, 2018	54,738	54,738
Adjustment to beginning accumulated deficit from adoption of accounting standard update | ASU 2016-02 [Member]	$ 6,564				6,564
Vesting of restricted stock units		$ 2	(2)
Vesting of restricted stock units, shares		131
Exercise of stock options	9,504	$ 2	9,502
Exercise of stock options, shares		189
Purchase of common stock under ESPP	1,231		1,231
Purchase of common stock under ESPP, shares		11
Stock-based compensation	32,341		32,341
Other comprehensive income (loss)	1,835			1,835
Net loss	(164,446)				(164,446)
Ending balance at Mar. 31, 2019	1,772,099	$ 551	3,430,030	(1,792)	(1,656,690)
Ending balance, shares at Mar. 31, 2019		55,069
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 547	3,386,958	(3,627)	(1,498,808)
Beginning balance, shares at Dec. 31, 2018	54,738	54,738
Exercise of stock options, shares	282
Other comprehensive income (loss)	$ 2,808
Net loss	(360,228)
Ending balance at Jun. 30, 2019	$ 1,636,374	$ 553	3,489,112	(819)	(1,852,472)
Ending balance, shares at Jun. 30, 2019	55,228	55,228
Beginning balance at Mar. 31, 2019	$ 1,772,099	$ 551	3,430,030	(1,792)	(1,656,690)
Beginning balance, shares at Mar. 31, 2019		55,069
Vesting of restricted stock units		$ 1	(1)
Vesting of restricted stock units, shares		66
Exercise of stock options	3,973	$ 1	3,972
Exercise of stock options, shares		93
Stock-based compensation	55,111		55,111
Other comprehensive income (loss)	973			973
Net loss	(195,782)				(195,782)
Ending balance at Jun. 30, 2019	$ 1,636,374	$ 553	$ 3,489,112	$ (819)	$ (1,852,472)
Ending balance, shares at Jun. 30, 2019	55,228	55,228