CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2018	$ 1,135,263,000	$ 342,000	$ 529,208,000	$ 646,214,000	$ (40,501,000)
Beginning Balance, shares at Dec. 31, 2018		170,820
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans	64,581,000	$ 6,000	64,575,000
Issuance of common stock under stock option plans, shares		3,018
Repurchase of common stock	(61,690,000)	$ (3,000)		(61,687,000)
Repurchase of common stock, shares		(1,398)
Stock-based compensation expense	45,589,000		45,589,000
Payment of dividends ($0.245 per common share)	(35,124,000)			(35,124,000)
Net income	203,865,000			203,865,000
Net unrealized gain (loss), net of tax of $(2,206), $981, and $515 in 2021, 2020, and 2019, 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	639,372,000	753,268,000	(37,275,000)
Balance, shares at Dec. 31, 2019		172,440
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans	125,715,000	$ 9,000	125,706,000
Issuance of common stock under stock option plans, shares		4,565
Repurchase of common stock	(51,036,000)	$ (2,000)		(51,034,000)
Repurchase of common stock, shares		(1,215)
Stock-based compensation expense	42,661,000		42,661,000
Payment of dividends ($0.245 per common share)	(390,508,000)			(390,508,000)
Net income	176,186,000			176,186,000
Net unrealized gain (loss), net of tax of $(2,206), $981, and $515 in 2021, 2020, and 2019, 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	807,739,000	487,912,000	(33,801,000)
Balance, shares at Dec. 31, 2020	175,790	175,790
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans	$ 63,292,000	$ 3,000	63,289,000
Issuance of common stock under stock option plans, shares	1,694	1,703
Repurchase of common stock	$ (161,652,000)	$ (4,000)		(161,648,000)
Repurchase of common stock, shares		(2,012)
Stock-based compensation expense	43,774,000		43,774,000
Payment of dividends ($0.245 per common share)	(43,263,000)			(43,263,000)
Net income	279,881,000			279,881,000
Net unrealized gain (loss), net of tax of $(2,206), $981, and $515 in 2021, 2020, and 2019, respectively	(7,152,000)				(7,152,000)
Net unrealized gain (loss) on available-for-sale investments, net of tax	(7,152,000)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(236,000)				(236,000)
Foreign currency translation adjustments, net of tax	(6,753,000)				(6,753,000)
Balance at Dec. 31, 2021	$ 1,430,093,000	$ 351,000	$ 914,802,000	$ 562,882,000	$ (47,942,000)
Balance, shares at Dec. 31, 2021	175,481	175,481