Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid in Capital	Accumulated Other Comprehensive (Loss) Gain	Accumulated Deficit
Begining balance at Dec. 31, 2017	$ 372,138	$ 89	$ 1,572,765	$ (1,850)	$ (1,198,866)
Begining balance (in shares) at Dec. 31, 2017		88,794
Net loss	(41,818)				(41,818)
Unrealized gain (loss) on available-for-sale investments, net of tax	(1,847)			(1,847)
Share-based compensation expense	19,879		19,879
Issuance of common stock for vested restricted stock units (in shares)		343
Issuance of common stock for stock option exercises	16,135	$ 1	16,134
Issuance of common stock for stock option exercises (in shares)		745
Ending balance at Mar. 31, 2018	364,487	$ 90	1,608,778	(3,697)	(1,240,684)
Ending balance (in shares) at Mar. 31, 2018		89,882
Begining balance at Dec. 31, 2018	480,765	$ 91	1,660,361	(1,932)	(1,177,755)
Begining balance (in shares) at Dec. 31, 2018		90,797
Net loss	(102,115)				(102,115)
Unrealized gain (loss) on available-for-sale investments, net of tax	1,699			1,699
Share-based compensation expense	15,764		15,764
Cumulative-effect adjustment to equity due to adoptionof ASU 2016-02 | ASU 2016-02	8,043				8,043
Issuance of common stock for vested restricted stock units (in shares)		353
Issuance of common stock for stock option exercises	2,581		2,581
Issuance of common stock for stock option exercises (in shares)		95
Issuance of common stock for employee stock purchase plan	2,538		2,538
Issuance of common stock for employee stock purchase, (in shares)		39
Ending balance at Mar. 31, 2019	$ 409,275	$ 91	$ 1,681,244	$ (233)	$ (1,271,827)
Ending balance (in shares) at Mar. 31, 2019		91,284