CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning Balance, Value at Dec. 31, 2017	$ 2,582,699	$ 117,515	$ 2,700,214	$ 28,689	$ 28,689	$ 483,733	$ 483,733	$ (386,322)	$ (386,322)	$ 2,456,599	$ 117,515	$ 2,574,114
Beginning Balance, Shares at Dec. 31, 2017				286,895	286,895
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
Net income	$ 344,255									344,255
Net unrealized holding (loss) gain on available-for-sale securities	(78)					(21)				(57)
Common stock issued for employee stock benefit plans	40,167			$ 310		39,857
Common stock issued for employee stock benefit plans, Shares				3,096
Common stock issued for agency acquisitions	100,000			$ 338		99,662
Common stock issued for agency acquisitions, Shares				3,376
Purchase of treasury stock	(100,000)					(8,750)		(91,250)
Common stock issued to directors	700			$ 1		699
Common stock issued to directors, Shares				13
Cash dividends paid	(84,690)									(84,690)
Ending Balance, Value at Dec. 31, 2018	3,000,568			$ 29,338		615,180		(477,572)		2,833,622
Ending Balance, Shares at Dec. 31, 2018				293,380
Net income	398,514									398,514
Net unrealized holding (loss) gain on available-for-sale securities	152					182				(30)
Common stock issued for employee stock benefit plans	60,180			$ 313		59,867
Common stock issued for employee stock benefit plans, Shares				3,129
Common stock issued for agency acquisitions	20,000			$ 57		19,943
Common stock issued for agency acquisitions, Shares				569
Purchase of treasury stock	(38,671)					20,000		(58,671)
Common stock issued to directors	880			$ 3		877
Common stock issued to directors, Shares				28
Cash dividends paid	(91,344)									(91,344)
Ending Balance, Value at Dec. 31, 2019	$ 3,350,279			$ 29,711		716,049		(536,243)		3,140,762
Ending Balance, Shares at Dec. 31, 2019	281,655			297,106
Net income	$ 480,483									480,483
Net unrealized holding (loss) gain on available-for-sale securities	496					466				30
Common stock issued for employee stock benefit plans	47,945			$ 184		47,761
Common stock issued for employee stock benefit plans, Shares				1,844
Common stock issued for agency acquisitions	30,120			$ 72		30,048
Common stock issued for agency acquisitions, Shares				723
Purchase of treasury stock	(55,095)							(55,095)
Common stock issued to directors	587			$ 2		585
Common stock issued to directors, Shares				16
Cash dividends paid	(100,592)									(100,592)
Ending Balance, Value at Dec. 31, 2020	$ 3,754,223			$ 29,969		$ 794,909		$ (591,338)		$ 3,520,683
Ending Balance, Shares at Dec. 31, 2020	283,004			299,689