CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands		Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock [Member]		Additional Paid-in Capital [Member]	[1]	Retained Earnings [Member]	[1]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2017		$ 1,095,673,000	$ (5,961,000)	$ 347,000	[1]	$ 461,338,000		$ 668,587,000		$ (5,961,000)	$ (34,599,000)
Beginning Balance, shares at Dec. 31, 2017	[1]			173,507
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans		26,783,000		$ 3,000	[1]	26,780,000
Issuance of common stock under stock option plans, shares	[1]			1,493
Repurchase of common stock		(203,822,000)		$ (8,000)	[1]			(203,814,000)
Repurchase of common stock, shares	[1]			(4,180)
Stock-based compensation expense		41,090,000				41,090,000
Payment of dividends		(31,865,000)						(31,865,000)
Net income		219,267,000						219,267,000
Net unrealized gain (loss), net of tax of $981, $515, and ($188) in 2020 2019, and 2018, respectively		(1,185,000)									(1,185,000)
Net unrealized gain (loss) on available-for-sale investments, net of tax		(1,185,000)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments		(501,000)									(501,000)
Foreign currency translation adjustments, net of tax		(4,216,000)									(4,216,000)
Balance at Dec. 31, 2018		1,135,263,000		$ 342,000	[1]	529,208,000		646,214,000			(40,501,000)
Balance, shares at Dec. 31, 2018	[1]			170,820
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans		64,581,000		$ 6,000	[1]	64,575,000
Issuance of common stock under stock option plans, shares	[1]			3,018
Repurchase of common stock		(61,690,000)		$ (3,000)	[1]			(61,687,000)
Repurchase of common stock, shares	[1]			(1,398)
Stock-based compensation expense		45,589,000				45,589,000
Payment of dividends		(35,124,000)						(35,124,000)
Net income		203,865,000						203,865,000
Net unrealized gain (loss), net of tax of $981, $515, and ($188) in 2020 2019, and 2018, respectively		5,219,000									5,219,000
Net unrealized gain (loss) on available-for-sale investments, net of tax		5,219,000
Reclassification of net realized (gain) loss on the sale of available-for-sale investments		(1,452,000)									(1,452,000)
Foreign currency translation adjustments, net of tax		(541,000)									(541,000)
Balance at Dec. 31, 2019		$ 1,355,710,000		$ 345,000	[1]	639,372,000		753,268,000			(37,275,000)
Balance, shares at Dec. 31, 2019		172,440		172,440	[1]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans		$ 125,715,000		$ 9,000	[1]	125,706,000
Issuance of common stock under stock option plans, shares		4,565		4,565	[1]
Repurchase of common stock		$ (51,036,000)		$ (2,000)	[1]			(51,034,000)
Repurchase of common stock, shares	[1]			(1,215)
Stock-based compensation expense		42,661,000				42,661,000
Payment of dividends		(390,508,000)						(390,508,000)
Net income		176,186,000
Net unrealized gain (loss), net of tax of $981, $515, and ($188) in 2020 2019, and 2018, respectively		6,478,000									6,478,000
Net unrealized gain (loss) on available-for-sale investments, net of tax		6,478,000
Reclassification of net realized (gain) loss on the sale of available-for-sale investments		(4,119,000)									(4,119,000)
Foreign currency translation adjustments, net of tax		1,115,000									1,115,000
Balance at Dec. 31, 2020		$ 1,262,202,000		$ 352,000	[1]	$ 807,739,000		$ 487,912,000			$ (33,801,000)
Balance, shares at Dec. 31, 2020		175,790		175,790	[1]

[1] Prior period amounts have been adjusted to reflect the two-for-one stock split effected in the form of a stock dividend which occurred in the fourth quarter of 2017.