Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		169,893,324
Beginning Balance 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)		1,327,017
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	(38,514)	$ 14	(38,528)
Issuance of common stock under employee stock purchase plan (in shares)		543,520
Issuance of common stock under employee stock purchase plan	25,768	$ 5	25,763
Stock-based compensation	108,893		108,893
Equity component of convertible senior notes, net of issuance costs of $1,649	275,836		275,836
Issuance costs for convertible senior notes	1,649
Issuance of warrants related to convertible senior notes	119,945		119,945
Purchase of note hedge related to convertible senior notes	(261,740)		(261,740)
Repurchases of common stock (in shares)		(2,452,768)
Repurchases of common stock	(185,512)			(185,512)
Net income	96,775					96,775
Foreign currency translation adjustments	(18,275)				(18,276)
Change in unrealized gain (loss) on investments, net of income tax (provision) benefit of $(547), $(299), $(1,101) and $572 for the three and six months ended June 30, 2019 and 2018, respectively	(1,765)				(1,765)
Ending balance (in shares) at Jun. 30, 2018		169,311,093
Ending Balance at Jun. 30, 2018	3,445,278	$ 1,718	4,303,531	(185,512)	(41,971)	(632,488)
Beginning balance (in shares) at Mar. 31, 2018		170,548,139
Beginning Balance at Mar. 31, 2018	3,386,258	$ 1,708	4,098,218	(19,785)	(18,334)	(675,549)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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)		378,583
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	(10,989)	$ 5	(10,994)
Issuance of common stock under employee stock purchase plan (in shares)		543,520
Issuance of common stock under employee stock purchase plan	25,768	$ 5	25,763
Stock-based compensation	56,503		56,503
Equity component of convertible senior notes, net of issuance costs of $1,649	275,836		275,836
Issuance costs for convertible senior notes	1,649
Issuance of warrants related to convertible senior notes	119,945		119,945
Purchase of note hedge related to convertible senior notes	(261,740)		(261,740)
Repurchases of common stock (in shares)		(2,159,149)
Repurchases of common stock	(165,727)			(165,727)
Treasury stock retirement	0
Net income	43,061					43,061
Foreign currency translation adjustments	(24,558)				(24,558)
Change in unrealized gain (loss) on investments, net of income tax (provision) benefit of $(547), $(299), $(1,101) and $572 for the three and six months ended June 30, 2019 and 2018, respectively	921				921
Ending balance (in shares) at Jun. 30, 2018		169,311,093
Ending Balance at Jun. 30, 2018	3,445,278	$ 1,718	4,303,531	(185,512)	(41,971)	(632,488)
Beginning balance (in shares) at Dec. 31, 2018		162,904,550
Beginning Balance 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)		1,528,391
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	(48,911)	$ 15	(48,926)
Issuance of common stock under employee stock purchase plan (in shares)		473,462
Issuance of common stock under employee stock purchase plan	27,669	$ 5	27,664
Stock-based compensation	112,069		112,069
Repurchases of common stock (in shares)		(1,547,312)
Repurchases of common stock	(116,247)			(116,247)
Net income	221,045					221,045
Foreign currency translation adjustments	3,351				3,351
Change in unrealized gain (loss) on investments, net of income tax (provision) benefit of $(547), $(299), $(1,101) and $572 for the three and six months ended June 30, 2019 and 2018, respectively	3,077				3,077
Ending balance (in shares) at Jun. 30, 2019		163,359,091
Ending Balance at Jun. 30, 2019	3,394,764	$ 1,649	3,760,840	(116,247)	(42,484)	(208,994)
Beginning balance (in shares) at Mar. 31, 2019		163,503,258
Beginning Balance at Mar. 31, 2019	3,283,217	$ 1,640	3,686,337	(34,872)	(46,979)	(322,909)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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)		437,733
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	(11,147)	$ 4	(11,151)
Issuance of common stock under employee stock purchase plan (in shares)		473,462
Issuance of common stock under employee stock purchase plan	27,669	$ 5	27,664
Stock-based compensation	57,990		57,990
Repurchases of common stock (in shares)		(1,055,362)
Repurchases of common stock	(81,375)			(81,375)
Net income	113,915					113,915
Foreign currency translation adjustments	2,849				2,849
Change in unrealized gain (loss) on investments, net of income tax (provision) benefit of $(547), $(299), $(1,101) and $572 for the three and six months ended June 30, 2019 and 2018, respectively	1,646				1,646
Ending balance (in shares) at Jun. 30, 2019		163,359,091
Ending Balance at Jun. 30, 2019	$ 3,394,764	$ 1,649	$ 3,760,840	$ (116,247)	$ (42,484)	$ (208,994)