Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Total	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		169,893,324
Balance as of January 1, 2019 at Dec. 31, 2017	$ 3,362,469	$ 1,699	$ 4,073,362	$ 0	$ (21,930)	$ (690,662)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment to accumulated deficit related to adoption of new accounting pronouncement	(38,601)					(38,601)
Issuance of common stock upon the exercise of stock options and vesting of restricted and deferred stock units, net of shares withheld for employee taxes (in shares)		948,434
Issuance of common stock upon the exercise of stock options and vesting of restricted and deferred stock units, net of shares withheld for employee taxes	(27,525)	$ 9	(27,534)
Stock-based compensation	52,390		52,390
Repurchases of common stock (in shares)		(293,619)
Repurchases of common stock	(19,785)			(19,785)
Net income	53,714					53,714
Foreign currency translation adjustments	6,282
Change in unrealized gain (loss) on investments, net of income tax (provision) benefit of $(554) and $871 for the three months ended March 31, 2019 and 2018, respectively	(2,686)
Ending balance (in shares) at Mar. 31, 2018		170,548,139
Balance as of March 31, 2019 at Mar. 31, 2018	3,386,258	$ 1,708	4,098,218	(19,785)	(18,334)	(675,549)
Beginning balance (in shares) at Dec. 31, 2018		162,904,550
Balance as of January 1, 2019 at Dec. 31, 2018	3,191,860	$ 1,629	3,670,033	0	(48,912)	(430,890)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment to accumulated deficit related to adoption of new accounting pronouncement	851					851
Issuance of common stock upon the exercise of stock options and vesting of restricted and deferred stock units, net of shares withheld for employee taxes (in shares)		1,090,658
Issuance of common stock upon the exercise of stock options and vesting of restricted and deferred stock units, net of shares withheld for employee taxes	(37,764)	$ 11	(37,775)
Stock-based compensation	54,079		54,079
Repurchases of common stock (in shares)		(491,950)
Repurchases of common stock	(34,872)			(34,872)
Net income	107,130					107,130
Foreign currency translation adjustments	502
Change in unrealized gain (loss) on investments, net of income tax (provision) benefit of $(554) and $871 for the three months ended March 31, 2019 and 2018, respectively	1,431
Ending balance (in shares) at Mar. 31, 2019		163,503,258
Balance as of March 31, 2019 at Mar. 31, 2019	$ 3,283,217	$ 1,640	$ 3,686,337	$ (34,872)	$ (46,979)	$ (322,909)