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, 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	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	55,069