CONDENSED CONSOLIDATED STATEMENTS OF EQUITY (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	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		293,380
Net income	113,896				113,896
Net unrealized holding (loss) gain on available-for- sale securities	106		106
Common stock issued for employee stock benefit plans	6,209	$ 246	5,963
Common stock issued for employee stock benefit plans, Shares		2,465
Cash dividends paid	(22,348)				(22,348)
Ending Balance, Value at Mar. 31, 2019	3,098,431	$ 29,584	621,249	(477,572)	2,925,170
Ending Balance, Shares at Mar. 31, 2019		295,845
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		293,380
Net income	321,995
Ending Balance, Value at Sep. 30, 2019	3,314,523	$ 29,714	703,949	(507,314)	3,088,174
Ending Balance, Shares at Sep. 30, 2019		297,136
Beginning Balance, Value at Mar. 31, 2019	3,098,431	$ 29,584	621,249	(477,572)	2,925,170
Beginning Balance, Shares at Mar. 31, 2019		295,845
Net income	92,593				92,593
Net unrealized holding (loss) gain on available-for- sale securities	181		205		(24)
Common stock issued for employee stock benefit plans	11,079	$ (5)	11,084
Common stock issued for employee stock benefit plans, Shares		(54)
Purchase of treasury stock			20,000	(20,000)
Common stock issued to directors	880	$ 3	877
Common stock issued to directors, Shares		28
Cash dividends paid	(22,533)				(22,533)
Ending Balance, Value at Jun. 30, 2019	3,180,631	$ 29,582	653,415	(497,572)	2,995,206
Ending Balance, Shares at Jun. 30, 2019		295,819
Net income	115,506				115,506
Net unrealized holding (loss) gain on available-for- sale securities	21		8		13
Common stock issued for employee stock benefit plans	30,658	$ 75	30,583
Common stock issued for employee stock benefit plans, Shares		748
Common stock issued for agency acquisitions	20,000	$ 57	19,943
Common stock issued for agency acquisitions, Shares		569
Purchase of treasury stock	(9,742)			(9,742)
Cash dividends paid	(22,551)				(22,551)
Ending Balance, Value at Sep. 30, 2019	3,314,523	$ 29,714	703,949	(507,314)	3,088,174
Ending Balance, Shares at Sep. 30, 2019		297,136
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	297,106
Net income	$ 152,400				152,400
Net unrealized holding (loss) gain on available-for- sale securities	293		194		99
Common stock issued for employee stock benefit plans	2,200	$ 182	2,018
Common stock issued for 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, 2019	$ 3,350,279	$ 29,711	716,049	(536,243)	3,140,762
Beginning Balance, Shares at Dec. 31, 2019	281,655	297,106
Net income	$ 383,163
Common stock issued for agency acquisitions	15,120
Ending Balance, Value at Sep. 30, 2020	$ 3,702,010	$ 29,937	763,749	(543,556)	3,451,880
Ending Balance, Shares at Sep. 30, 2020	283,745	299,371
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		298,934
Net income	96,784				96,784
Net unrealized holding (loss) gain on available-for- sale securities	315		384		(69)
Common stock issued for employee stock benefit plans	20,075	$ (7)	20,082
Common stock issued for 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		299,151
Net income	133,979				133,979
Net unrealized holding (loss) gain on available-for- sale securities	(51)		(51)
Common stock issued for employee stock benefit plans	9,417	$ 10	9,407
Common stock issued for employee stock benefit plans, Shares		106
Common stock issued for agency acquisitions	5,120	$ 12	5,108
Common stock issued for agency acquisitions, Shares		114
Purchase of treasury stock	(5,884)			(5,884)
Cash dividends paid	(24,089)				(24,089)
Ending Balance, Value at Sep. 30, 2020	$ 3,702,010	$ 29,937	$ 763,749	$ (543,556)	$ 3,451,880
Ending Balance, Shares at Sep. 30, 2020	283,745	299,371