Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 789,217	$ 6	$ 2,006,598	$ (379)	$ (1,217,008)
Balance (in shares) at Dec. 31, 2017		64,792
Exercise of options for common stock	11,888	$ 1	11,887
Exercise of options for common stock, shares		653
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		14
Issuance of common stock under employee stock purchase plan	756		756
Issuance of common stock under employee stock purchase plan, shares		34
Stock-based compensation	10,526		10,526
Unrealized gains (losses) from available-for-sale securities	(264)			(264)
Net loss	(35,363)				(35,363)
Balance at Mar. 31, 2018	776,760	$ 7	2,029,767	(643)	(1,252,371)
Balance (in shares) at Mar. 31, 2018		65,493
Balance at Dec. 31, 2017	789,217	$ 6	2,006,598	(379)	(1,217,008)
Balance (in shares) at Dec. 31, 2017		64,792
Net loss	(221,023)
Balance at Sep. 30, 2018	639,848	$ 7	2,077,864	8	(1,438,031)
Balance (in shares) at Sep. 30, 2018		66,693
Balance at Mar. 31, 2018	776,760	$ 7	2,029,767	(643)	(1,252,371)
Balance (in shares) at Mar. 31, 2018		65,493
Exercise of options for common stock	21,743		21,743
Exercise of options for common stock, shares		872
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		26
Shares withheld for taxes	(5,750)		(5,750)
Shares withheld for taxes, shares		(45)
Stock-based compensation	15,279		15,279
Unrealized gains (losses) from available-for-sale securities	282			282
Net loss	(109,267)				(109,267)
Balance at Jun. 30, 2018	699,047	$ 7	2,061,039	(361)	(1,361,638)
Balance (in shares) at Jun. 30, 2018		66,346
Exercise of options for common stock	7,094		7,094
Exercise of options for common stock, shares		323
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		10
Shares withheld for taxes	(3,303)		(3,303)
Shares withheld for taxes, shares		(27)
Issuance of common stock under employee stock purchase plan	1,550		1,550
Issuance of common stock under employee stock purchase plan, shares		41
Stock-based compensation	11,484		11,484
Unrealized gains (losses) from available-for-sale securities	369			369
Net loss	(76,393)				(76,393)
Balance at Sep. 30, 2018	639,848	$ 7	2,077,864	8	(1,438,031)
Balance (in shares) at Sep. 30, 2018		66,693
Balance at Dec. 31, 2018	1,032,276	$ 7	2,611,294	(99)	(1,578,926)
Balance (in shares) at Dec. 31, 2018		71,072
Exercise of options for common stock	9,973		9,973
Exercise of options for common stock, shares		382
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		35
Shares withheld for taxes	(889)		(889)
Shares withheld for taxes, shares		(7)
Issuance of common stock for cash, net of offering costs	365,264		365,264
Issuance of common stock for cash, net of offering costs, shares		2,604
Issuance of common stock under employee stock purchase plan	2,326		2,326
Issuance of common stock under employee stock purchase plan, shares		48
Stock-based compensation	16,139		16,139
Unrealized gains (losses) from available-for-sale securities	118			118
Net loss	(76,643)				(76,643)
Balance at Mar. 31, 2019	1,348,564	$ 7	3,004,107	19	(1,655,569)
Balance (in shares) at Mar. 31, 2019		74,134
Balance at Dec. 31, 2018	1,032,276	$ 7	2,611,294	(99)	(1,578,926)
Balance (in shares) at Dec. 31, 2018		71,072
Net loss	(479,372)
Balance at Sep. 30, 2019	995,204	$ 7	3,053,420	75	(2,058,298)
Balance (in shares) at Sep. 30, 2019		74,505
Balance at Mar. 31, 2019	1,348,564	$ 7	3,004,107	19	(1,655,569)
Balance (in shares) at Mar. 31, 2019		74,134
Exercise of options for common stock	7,092		7,092
Exercise of options for common stock, shares		176
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		18
Issuance of common stock for cash, net of offering costs	89		89
Stock-based compensation	19,762		19,762
Unrealized gains (losses) from available-for-sale securities	63			63
Net loss	(276,403)				(276,403)
Balance at Jun. 30, 2019	1,099,167	$ 7	3,031,050	82	(1,931,972)
Balance (in shares) at Jun. 30, 2019		74,328
Exercise of options for common stock	2,428		2,428
Exercise of options for common stock, shares		166
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		1
Shares withheld for taxes	(3,448)		(3,448)
Shares withheld for taxes, shares		(34)
Issuance of common stock under employee stock purchase plan	2,753		2,753
Issuance of common stock under employee stock purchase plan, shares		44
Stock-based compensation	20,637		20,637
Unrealized gains (losses) from available-for-sale securities	(7)			(7)
Net loss	(126,326)				(126,326)
Balance at Sep. 30, 2019	$ 995,204	$ 7	$ 3,053,420	$ 75	$ (2,058,298)
Balance (in shares) at Sep. 30, 2019		74,505