Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Deferred Compensation [Member]	Accumulated Other Comprehensive Earnings (Losses) [Member]
Beginning Balance at Dec. 31, 2019	$ 1,227,197	$ 1,359	$ 450,676	$ 707,656	$ (8,222)	$ 8,222	$ 67,506
Beginning Balance, Shares at Dec. 31, 2019		135,891,755
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2019					(927,408)
Stock issued in acquisition of TB&T Bancshares, Inc.	220,273	$ 63	220,210
Stock issued in acquisition of TB&T Bancshares, Inc., Shares		6,275,574
Net earnings (Unaudited)	143,558			143,558
Stock option exercises (unaudited)	3,990	$ 2	3,988
Stock option exercises, Shares		246,919
Restricted stock grant/fortfeiture, net (unaudited)	913		913
Restricted stock grant/fortfeiture, net (unaudited), Shares		32,149
Cash dividends declared	(54,012)			(54,012)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	84,557						84,557
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (558)	558
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					(7,451)
Stock option expense (unaudited)	1,033		1,033
Shares repurchased under stock repurchase authorization (unaudited)	(8,008)	$ (3)	(8,005)
Shares repurchased under stock repurchase authorization (unaudited), Shares		(324,802)
Ending Balance at Sep. 30, 2020	$ 1,619,501	$ 1,421	668,815	797,202	$ (8,780)	8,780	152,063
Ending Balance, Shares at Sep. 30, 2020		142,121,595
Treasury Stock, Ending Balance, Shares at Sep. 30, 2020	(934,859)				(934,859)
Beginning Balance at Jun. 30, 2020	$ 1,582,449	$ 1,420	666,963	762,830	$ (8,697)	8,697	151,236
Beginning Balance, Shares at Jun. 30, 2020		142,035,396
Treasury Stock, Beginning Balance, Shares at Jun. 30, 2020					(932,018)
Net earnings (Unaudited)	52,857			52,857
Stock option exercises (unaudited)	1,509	$ 1	1,508
Stock option exercises, Shares		86,199
Cash dividends declared	(18,485)			18,485
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	827						827
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ 83	(83)
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					(2,841)
Stock option expense (unaudited)	344		344
Ending Balance at Sep. 30, 2020	$ 1,619,501	$ 1,421	668,815	797,202	$ (8,780)	8,780	152,063
Ending Balance, Shares at Sep. 30, 2020		142,121,595
Treasury Stock, Ending Balance, Shares at Sep. 30, 2020	(934,859)				(934,859)
Beginning Balance at Dec. 31, 2020	$ 1,678,190	$ 1,422	669,644	836,729	$ (9,126)	9,126	170,395
Beginning Balance, Shares at Dec. 31, 2020		142,161,834
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2020	(938,591)				(938,591)
Net earnings (Unaudited)	$ 172,225			172,225
Stock option exercises (unaudited)	4,676	$ 3	4,673
Stock option exercises, Shares		273,167
Restricted stock grant/fortfeiture, net (unaudited)	(165)		(165)
Restricted stock grant/fortfeiture, net (unaudited), Shares		32,686
Cash dividends declared	(61,230)			(61,230)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(60,948)						(60,948)
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (750)	750
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					788
Stock option expense (unaudited)	987		987
Ending Balance at Sep. 30, 2021	$ 1,733,735	$ 1,425	675,139	947,724	$ (9,876)	9,876	109,447
Ending Balance, Shares at Sep. 30, 2021		142,467,687
Treasury Stock, Ending Balance, Shares at Sep. 30, 2021	(937,803)				(937,803)
Beginning Balance at Jun. 30, 2021	$ 1,720,369	$ 1,424	672,288	910,171	$ (9,641)	9,641	136,486
Beginning Balance, Shares at Jun. 30, 2021		142,359,774
Treasury Stock, Beginning Balance, Shares at Jun. 30, 2021					(938,629)
Net earnings (Unaudited)	58,928			58,928
Stock option exercises (unaudited)	2,058	$ 1	2,057
Stock option exercises, Shares		107,796
Restricted stock grant/fortfeiture, net (unaudited)	498		498
Restricted stock grant/fortfeiture, net (unaudited), Shares		117
Cash dividends declared	(21,375)			(21,375)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(27,039)						(27,039)
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ 235	(235)
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					826
Stock option expense (unaudited)	296		296
Ending Balance at Sep. 30, 2021	$ 1,733,735	$ 1,425	$ 675,139	$ 947,724	$ (9,876)	$ 9,876	$ 109,447
Ending Balance, Shares at Sep. 30, 2021		142,467,687
Treasury Stock, Ending Balance, Shares at Sep. 30, 2021	(937,803)				(937,803)