Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative-effect adjustment to accumulated deficit	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Treasury Stock	Retained Earnings (Accumulated Deficit)	Retained Earnings (Accumulated Deficit) Cumulative-effect adjustment to accumulated deficit
Beginning balance (in shares) at Dec. 31, 2018			162,904,550
Beginning Balance at Dec. 31, 2018	$ 3,191,860	$ 851	$ 1,629	$ 3,670,033	$ (48,912)	$ 0	$ (430,890)	$ 851
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)			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 deferred tax of $23,170 and issuance costs of $2,880	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 (loss) 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	(56,424)	(291,788)	(71,104)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Deferred tax for convertible senior notes	23,170
Issuance costs for convertible senior notes	2,880
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	(42,484)	(116,247)	(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 deferred tax of $23,170 and issuance costs of $2,880	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 (loss) 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	(56,424)	(291,788)	(71,104)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Deferred tax for convertible senior notes	23,170
Issuance costs for convertible senior notes	2,880
Beginning balance (in shares) at Dec. 31, 2019			162,000,843
Beginning Balance at Dec. 31, 2019	3,657,958		$ 1,620	3,653,486	(45,144)	0	47,996
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)			1,667,888
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	(76,679)		$ 17	(76,696)
Issuance of common stock under employee stock purchase plan (in shares)			393,921
Issuance of common stock under employee stock purchase plan	29,170		$ 4	29,166
Stock-based compensation	175,272			175,272
Repurchases of common stock (in shares)			(1,262,868)
Repurchases of common stock	(121,078)					(121,078)
Net income	443,684
Foreign currency translation adjustments	(7,292)
Change in unrealized gain (loss) on investments, net of tax	6,582				6,582
Ending balance (in shares) at Sep. 30, 2020			162,799,784
Ending Balance at Sep. 30, 2020	4,107,617		$ 1,641	3,781,228	(45,854)	(121,078)	491,680
Beginning balance (in shares) at Jun. 30, 2020			162,630,477
Beginning Balance at Jun. 30, 2020	3,904,295		$ 1,638	3,734,787	(57,307)	(107,880)	333,057
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)			289,304
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	(13,387)		$ 3	(13,390)
Stock-based compensation	59,831			59,831
Repurchases of common stock (in shares)			(119,997)
Repurchases of common stock	(13,198)					(13,198)
Net income	158,623						158,623
Foreign currency translation adjustments	13,177				13,177
Change in unrealized gain (loss) on investments, net of tax	(1,724)				(1,724)
Ending balance (in shares) at Sep. 30, 2020			162,799,784
Ending Balance at Sep. 30, 2020	$ 4,107,617		$ 1,641	$ 3,781,228	$ (45,854)	$ (121,078)	$ 491,680