CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2014		76,134		(535)
Beginning balance at Dec. 31, 2014	$ 113,706	$ 755	$ 246,000	$ (652)	$ (528)	$ (131,869)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from follow-on public offering, net of issuance costs (in shares)		8,780
Issuance of common stock from follow-on public offering, net of issuance costs	190,096	$ 88	190,008
Issuance of common stock upon exercise of stock options (in shares)		3,275
Issuance of common stock upon exercise of stock options	10,613	$ 33	10,580
Issuance of common stock for settlement of restricted stock units (RSUs) (in shares)		1,655
Issuance of common stock for settlement of RSUs	(609)	$ 17	(626)
Vesting of early exercised stock options	1,046		1,046
Issuance of common stock in connection with employee stock purchase plan (in shares)		1,019
Issuance of common stock in connection with employee stock purchase plan	9,375	$ 12	9,363
Repurchase of common stock (in shares)		(2)
Share-based compensation	54,363		54,363
Tax benefit from share-based award activity	449		449
Other comprehensive (loss) gain, net of income taxes	(1,697)				(1,697)
Retirement of treasury stock	0
Net loss	(84,052)					(84,052)
Ending balance (in shares) at Dec. 31, 2015		90,861		(535)
Ending balance at Dec. 31, 2015	293,290	$ 905	511,183	$ (652)	(2,225)	(215,921)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		2,924
Issuance of common stock upon exercise of stock options	25,435	$ 29	25,406
Issuance of common stock for settlement of restricted stock units (RSUs) (in shares)		2,894
Issuance of common stock for settlement of RSUs	(803)	$ 29	(832)
Vesting of early exercised stock options	$ 629	$ 1	628
Issuance of common stock in connection with employee stock purchase plan (in shares)	600	554
Issuance of common stock in connection with employee stock purchase plan	$ 10,892	$ 7	10,885
Repurchase of common stock (in shares)		(38)
Share-based compensation	76,419		76,419
Tax benefit from share-based award activity	337		337
Other comprehensive (loss) gain, net of income taxes	(2,972)				(2,972)
Retirement of treasury stock	0
Net loss	(103,799)					(103,799)
Ending balance (in shares) at Dec. 31, 2016		97,195		(535)
Ending balance at Dec. 31, 2016	299,428	$ 971	624,026	$ (652)	(5,197)	(319,720)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment resulting from the adoption of new accounting policies | Accounting Standards Update 2016-09	0		493			(493)
Cumulative-effect adjustment resulting from the adoption of new accounting policies | Accounting Standards Update 2016-16	$ 265					265
Issuance of common stock upon exercise of stock options (in shares)	2,664	2,664
Issuance of common stock upon exercise of stock options	$ 31,882	$ 27	31,855
Issuance of common stock for settlement of restricted stock units (RSUs) (in shares)		3,145
Issuance of common stock for settlement of RSUs	(2,989)	$ 31	(3,020)
Vesting of early exercised stock options	$ 412	$ 1	411
Issuance of common stock in connection with employee stock purchase plan (in shares)	700	652
Issuance of common stock in connection with employee stock purchase plan	$ 12,910	$ 6	12,904
Share-based compensation	87,546		87,546
Other comprehensive (loss) gain, net of income taxes	2,825				2,825
Retirement of treasury stock (in shares)		(535)		(535)
Retirement of treasury stock	0	$ 5	647	$ 652
Net loss	(110,638)					(110,638)
Ending balance (in shares) at Dec. 31, 2017		103,121		0
Ending balance at Dec. 31, 2017	$ 321,641	$ 1,031	$ 753,568	$ 0	$ (2,372)	$ (430,586)