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, 2017	$ 2,582,699	$ 28,689	$ 483,733	$ (386,322)	$ 2,456,599
Beginning Balance, Shares at Dec. 31, 2017		286,895
Adoption of Topic 606 at January 1, 2018 (Accounting Standards Update 2014-09) at Dec. 31, 2017	117,515				117,515
Beginning Balance, Value at Dec. 31, 2017	2,700,214	$ 28,689	483,733	(386,322)	2,574,114
Net income	90,828				90,828
Net unrealized holding (loss) gain on available-for- sale securities	(132)		(77)		(55)
Common stock issued for employee stock benefit plans	(321)	$ 6	(327)
Common stock issued for employee stock benefit plans, Shares		53
Purchase of treasury stock			11,250	(11,250)
Common stock issued to directors	700	$ 1	699
Common stock issued to directors, Shares		13
Cash dividends paid	(20,696)				(20,696)
Ending Balance, Value at Mar. 31, 2018	2,770,593	$ 28,696	495,278	(397,572)	2,644,191
Ending Balance, Shares at Mar. 31, 2018		286,961
Beginning Balance, Value at Dec. 31, 2017	2,582,699	$ 28,689	483,733	(386,322)	2,456,599
Beginning Balance, Shares at Dec. 31, 2017		286,895
Adoption of Topic 606 at January 1, 2018 (Accounting Standards Update 2014-09) at Dec. 31, 2017	117,515				117,515
Beginning Balance, Value at Dec. 31, 2017	2,700,214	$ 28,689	483,733	(386,322)	2,574,114
Net income	270,803
Ending Balance, Value at Sep. 30, 2018	2,939,408	$ 29,011	525,465	(397,572)	2,782,504
Ending Balance, Shares at Sep. 30, 2018		290,109
Beginning Balance, Value at Mar. 31, 2018	2,770,593	$ 28,696	495,278	(397,572)	2,644,191
Beginning Balance, Shares at Mar. 31, 2018		286,961
Net income	73,922				73,922
Net unrealized holding (loss) gain on available-for- sale securities	(29)		(27)		(2)
Common stock issued for employee stock benefit plans	7,711	$ 12	7,699
Common stock issued for employee stock benefit plans, Shares		123
Cash dividends paid	(20,715)				(20,715)
Ending Balance, Value at Jun. 30, 2018	2,831,482	$ 28,708	502,950	(397,572)	2,697,396
Ending Balance, Shares at Jun. 30, 2018		287,084
Net income	106,053				106,053
Net unrealized holding (loss) gain on available-for- sale securities	(68)		(68)
Common stock issued for employee stock benefit plans	22,886	$ 303	22,583
Common stock issued for employee stock benefit plans, Shares		3,025
Cash dividends paid	(20,945)				(20,945)
Ending Balance, Value at Sep. 30, 2018	2,939,408	$ 29,011	525,465	(397,572)	2,782,504
Ending Balance, Shares at Sep. 30, 2018		290,109
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	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	279,583	293,380
Net income	$ 321,995
Common stock issued for agency acquisitions	20,000
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	282,498	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	282,498	297,136