CONDENSED CONSOLIDATED STATEMENTS OF EQUITY (UNAUDITED) - 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, 2019	$ 3,350,279	$ 29,711	$ 716,049	$ (536,243)		$ 3,140,762
Beginning Balance, Shares at Dec. 31, 2019		297,106
Net income	152,400					152,400
Net unrealized holding (loss) gain on available-for-sale debt securities	293		194			99
Common stock issued for stock incentive plan & employee stock benefit plans	2,200	$ 182	2,018
Common stock issued for stock incentive plan & employee stock benefit plans, Shares		1,828
Purchase of treasury stock	(1,429)			(1,429)
Cash dividends paid	(23,902)					(23,902)
Ending Balance, Value at Mar. 31, 2020	3,479,841	$ 29,893	718,261	(537,672)		3,269,359
Ending Balance, Shares at Mar. 31, 2020		298,934
Beginning Balance, Value at Dec. 31, 2020	$ 3,754,223	$ 29,969	794,909	(591,338)		3,520,683
Beginning Balance, Shares at Dec. 31, 2020	283,004	299,689
Net income	$ 199,744					199,744
Net unrealized holding (loss) gain on available-for-sale debt securities	(245)		(508)		$ 263
Foreign currency translation	(5,131)				(5,253)	122
Employee stock purchase plans	3,032		3,032
Common stock issued for stock incentive plan & employee stock benefit plans	15,646	$ 140	15,506
Common stock issued for stock incentive plan & employee stock benefit plans, Shares		1,400
Common stock issued for agency acquisitions	4,892	$ 11	4,881
Common stock issued for agency acquisitions, Shares		107
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(45,052)	$ (98)	(44,954)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(973)
Purchase of treasury stock	(70,020)			(70,020)
Cash dividends paid	(26,092)					(26,092)
Ending Balance, Value at Mar. 31, 2021	$ 3,830,997	$ 30,022	$ 772,866	$ (661,358)	$ (4,990)	$ 3,694,457
Ending Balance, Shares at Mar. 31, 2021	281,980	300,223