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		281,655
Net income	152,400					152,400
Net unrealized holding (loss) gain on available-for- sale securities	293		194			99
Employee stock purchase plan	2,191		2,191
Common stock issued for stock incentive plan & employee stock benefit plans	9	$ 182	(173)
Common stock issued for stock incentive plan & employee stock benefit plans, Shares		1,828
Purchase of treasury stock	(1,429)			(1,429)
Purchase of treasury stock, Shares		(42)
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		283,441
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		281,655
Net income	249,184
Ending Balance, Value at Jun. 30, 2020	3,583,518	$ 29,915	749,285	(537,672)		3,341,990
Ending Balance, Shares at Jun. 30, 2020		283,658
Beginning Balance, Value at Mar. 31, 2020	3,479,841	$ 29,893	718,261	(537,672)		3,269,359
Beginning Balance, Shares at Mar. 31, 2020		283,441
Net income	96,784					96,784
Net unrealized holding (loss) gain on available-for- sale securities	315		384			(69)
Employee stock purchase plan	1,349		1,349
Common stock issued for stock incentive plan & employee stock benefit plans	18,726	$ (7)	18,733
Common stock issued for stock incentive plan & employee stock benefit plans, Shares		(73)
Common stock issued for agency acquisitions	10,000	$ 27	9,973
Common stock issued for agency acquisitions, Shares		274
Common stock issued to directors	587	$ 2	585
Common stock issued to directors, Shares		16
Cash dividends paid	(24,084)					(24,084)
Ending Balance, Value at Jun. 30, 2020	3,583,518	$ 29,915	749,285	(537,672)		3,341,990
Ending Balance, Shares at Jun. 30, 2020		283,658
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	283,004
Net income	$ 199,744					199,744
Net unrealized holding (loss) gain on available-for- sale securities	(245)		(508)		$ 263
Foreign currency translation	(5,131)				(5,253)	122
Employee stock purchase plan	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)
Purchase of treasury stock, Shares		(1,558)
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
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	283,004
Net income	$ 339,033
Common stock issued for agency acquisitions	4,892
Ending Balance, Value at Jun. 30, 2021	$ 3,946,002	$ 30,010	784,620	(672,763)	(3,549)	3,807,684
Ending Balance, Shares at Jun. 30, 2021	281,621	281,621
Beginning Balance, Value at Mar. 31, 2021	$ 3,830,997	$ 30,022	772,866	(661,358)	(4,990)	3,694,457
Beginning Balance, Shares at Mar. 31, 2021		281,980
Net income	139,290					139,290
Net unrealized holding (loss) gain on available-for- sale securities	(30)				(30)
Foreign currency translation	1,471				1,471
Employee stock purchase plan	1,988		1,988
Common stock issued for stock incentive plan & employee stock benefit plans	12,059	$ (8)	12,067
Common stock issued for stock incentive plan & employee stock benefit plans, Shares		(83)
Common stock issued to directors	899	$ 2	897
Common stock issued to directors, Shares		17
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(3,204)	$ (6)	(3,198)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(61)
Purchase of treasury stock	(11,405)			(11,405)
Purchase of treasury stock, Shares		(232)
Cash dividends paid	(26,063)					(26,063)
Ending Balance, Value at Jun. 30, 2021	$ 3,946,002	$ 30,010	$ 784,620	$ (672,763)	$ (3,549)	$ 3,807,684
Ending Balance, Shares at Jun. 30, 2021	281,621	281,621