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)		1,797,241
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	(44,601)	$ 18	(44,619)
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	164,310		164,310
Equity component of convertible senior notes, net of issuance costs	275,836		275,836
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)		(8,347,366)
Repurchases of common stock	(625,925)			(625,925)
Net income	204,358					204,358
Foreign currency translation adjustments					(26,047)
Change in unrealized gain on investments, net of tax	(242)				(241)
Ending balance (in shares) at Sep. 30, 2018		163,886,719
Ending Balance at Sep. 30, 2018	3,155,531	$ 1,722	4,352,857	(625,925)	(48,218)	(524,905)
Beginning balance (in shares) at Jun. 30, 2018		169,311,093
Beginning Balance at Jun. 30, 2018	3,445,278	$ 1,718	4,303,531	(185,512)	(41,971)	(632,488)
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)		470,224
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	(6,087)	$ 4	(6,091)
Stock-based compensation	55,417		55,417
Repurchases of common stock (in shares)		(5,894,598)
Repurchases of common stock	(440,413)			(440,413)
Net income	107,583					107,583
Foreign currency translation adjustments	(7,771)				(7,771)
Change in unrealized gain on investments, net of tax	1,524				1,524
Ending balance (in shares) at Sep. 30, 2018		163,886,719
Ending Balance at Sep. 30, 2018	3,155,531	$ 1,722	4,352,857	(625,925)	(48,218)	(524,905)
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)		1,833,758
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	(59,635)	$ 18	(59,653)
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	167,475		167,475
Equity component of convertible senior notes, net of issuance costs	240,820		240,820
Issuance of warrants related to convertible senior notes	185,150		185,150
Purchase of note hedge related to convertible senior notes	(312,225)		(312,225)
Repurchases of common stock (in shares)		(3,552,268)
Repurchases of common stock	(291,788)			(291,788)
Net income	358,935					358,935
Foreign currency translation adjustments	(10,744)				(10,744)
Change in unrealized gain on investments, net of tax	3,232				3,232
Ending balance (in shares) at Sep. 30, 2019		161,659,502
Ending Balance at Sep. 30, 2019	3,501,600	$ 1,652	3,919,264	(291,788)	(56,424)	(71,104)
Beginning balance (in shares) at Jun. 30, 2019		163,359,091
Beginning Balance at Jun. 30, 2019	3,394,764	$ 1,649	3,760,840	(116,247)	(42,484)	(208,994)
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)		305,367
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	(10,724)	$ 3	(10,727)
Stock-based compensation	55,406		55,406
Equity component of convertible senior notes, net of issuance costs	240,820		240,820
Issuance of warrants related to convertible senior notes	185,150		185,150
Purchase of note hedge related to convertible senior notes	(312,225)		(312,225)
Repurchases of common stock (in shares)		(2,004,956)
Repurchases of common stock	(175,541)			(175,541)
Net income	137,890					137,890
Foreign currency translation adjustments	(14,095)				(14,095)
Change in unrealized gain on investments, net of tax	155				155
Ending balance (in shares) at Sep. 30, 2019		161,659,502
Ending Balance at Sep. 30, 2019	$ 3,501,600	$ 1,652	$ 3,919,264	$ (291,788)	$ (56,424)	$ (71,104)