CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Retained Earnings
Beginning Balance, Value at Dec. 31, 2018	$ 3,000,568	$ 29,338	$ 615,180	$ (477,572)		$ 2,833,622
Beginning Balance, Shares at Dec. 31, 2018		279,583
Net income	398,514					398,514
Net unrealized holding (loss) gain on available-for-sale securities	152		182			(30)
Employee stock purchase plan	30,551	$ 98	30,453
Employee stock purchase plan, Shares		976
Stock incentive plans	40,563	$ 252	40,311
Stock incentive plans, Shares		2,519
Agency acquisition	20,000	$ 57	19,943
Agency acquisition, Shares		569
Directors	880	$ 3	877
Directors, Shares		28
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(10,934)	$ (37)	(10,897)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(366)
Purchase of treasury stock	(38,671)		20,000	(58,671)
Purchase of treasury stock, Shares		(1,654)
Cash dividends paid	(91,344)					(91,344)
Ending Balance, Value at Dec. 31, 2019	3,350,279	$ 29,711	716,049	(536,243)	$ 0	3,140,762
Ending Balance, Shares at Dec. 31, 2019		281,655
Net income	480,483					480,483
Net unrealized holding (loss) gain on available-for-sale securities	496		466			30
Employee stock purchase plan	38,143	$ 96	38,047
Employee stock purchase plan, Shares		962
Stock incentive plans	51,123	$ 189	50,934
Stock incentive plans, Shares		1,895
Agency acquisition	30,120	$ 72	30,048
Agency acquisition, Shares		723
Directors	587	$ 2	585
Directors, Shares		16
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(41,321)	$ (101)	(41,220)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(1,013)
Purchase of treasury stock	(55,095)			(55,095)
Purchase of treasury stock, Shares		(1,234)
Cash dividends paid	(100,592)					(100,592)
Ending Balance, Value at Dec. 31, 2020	$ 3,754,223	$ 29,969	794,909	(591,338)	0	3,520,683
Ending Balance, Shares at Dec. 31, 2020	283,004	283,004
Net income	$ 587,104					587,104
Net unrealized holding (loss) gain on available-for-sale securities	(630)		(508)		(122)
Foreign currency translation	(9,164)				(9,287)	123
Employee stock purchase plan	42,885	$ 85	42,800
Employee stock purchase plan, Shares		851
Stock incentive plans	51,260	$ 131	51,129
Stock incentive plans, Shares		1,313
Agency acquisition	9,892	$ 19	9,873
Agency acquisition, Shares		184
Directors	899	$ 2	897
Directors, Shares		17
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(49,783)	$ (107)	(49,676)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(1,061)
Purchase of treasury stock	(82,564)			(82,564)
Purchase of treasury stock, Shares		(1,812)
Cash dividends paid	(107,229)					(107,229)
Ending Balance, Value at Dec. 31, 2021	$ 4,196,893	$ 30,099	$ 849,424	$ (673,902)	$ (9,409)	$ 4,000,681
Ending Balance, Shares at Dec. 31, 2021	282,496	282,496