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 [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 gain (loss) from cash equivalents and 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, 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 gain (loss) from cash equivalents and 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