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)	90,701			90,701
Stock option exercises (unaudited)	2,481	$ 1	2,480
Stock option exercises, Shares		160,720
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited)	913		913
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited), Shares		32,149
Cash dividends declared	(35,527)			(35,527)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	83,730						83,730
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)	(8,008)	$ (3)	(8,005)		$ (475)	475
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares		(324,802)			(4,610)
Stock option expense (unaudited)	689		689
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 Jun. 30, 2020	$ 1,582,449	$ 1,420	666,963	762,830	$ (8,697)	8,697	151,236
Ending Balance, Shares at Jun. 30, 2020		142,035,396
Treasury Stock, Ending Balance, Shares at Jun. 30, 2020	(932,018)				(932,018)
Beginning Balance at Mar. 31, 2020	$ 1,526,362	$ 1,423	673,535	727,828	$ (8,437)	8,437	123,576
Beginning Balance, Shares at Mar. 31, 2020		142,314,930
Treasury Stock, Beginning Balance, Shares at Mar. 31, 2020					(928,417)
Net earnings (unaudited)	53,470			53,470
Stock option exercises (unaudited)	289		289
Stock option exercises, Shares		16,532
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited)	795		795
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited), Shares		28,736
Cash dividends declared	(18,468)			(18,468)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	27,660						27,660
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (260)	260
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					(3,601)
Stock option expense (unaudited)	349		349
Ending Balance at Jun. 30, 2020	$ 1,582,449	$ 1,420	666,963	762,830	$ (8,697)	8,697	151,236
Ending Balance, Shares at Jun. 30, 2020		142,035,396
Treasury Stock, Ending Balance, Shares at Jun. 30, 2020	(932,018)				(932,018)
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)	$ 113,297			113,297
Stock option exercises (unaudited)	2,618	$ 2	2,616
Stock option exercises, Shares		165,371
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited)	(663)		(663)
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited), Shares		32,569
Cash dividends declared	(39,855)			(39,855)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(33,909)						(33,909)
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (515)	515
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					(38)
Stock option expense (unaudited)	691		691
Shares repurchased under stock repurchase authorization (unaudited), Shares		324,802
Ending Balance at Jun. 30, 2021	$ 1,720,369	$ 1,424	672,288	910,171	$ (9,641)	9,641	136,486
Ending Balance, Shares at Jun. 30, 2021		142,359,774
Treasury Stock, Ending Balance, Shares at Jun. 30, 2021	(938,629)				(938,629)
Beginning Balance at Mar. 31, 2021	$ 1,665,428	$ 1,423	671,849	875,147	$ (9,385)	9,385	117,009
Beginning Balance, Shares at Mar. 31, 2021		142,285,611
Treasury Stock, Beginning Balance, Shares at Mar. 31, 2021					(938,004)
Net earnings (unaudited)	56,379			56,379
Stock option exercises (unaudited)	714	$ 1	713
Stock option exercises, Shares		40,847
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited)	(646)		(646)
Restricted stock grant (unaudited) Restricted stock grant/fortfeiture, net (unaudited), Shares		33,316
Cash dividends declared	(21,355)			(21,355)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	19,477						19,477
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (256)	256
Shares purchased in connection with directors' deferred compensation plan, net (unaudited), Shares					(625)
Stock option expense (unaudited)	372		372
Ending Balance at Jun. 30, 2021	$ 1,720,369	$ 1,424	$ 672,288	$ 910,171	$ (9,641)	$ 9,641	$ 136,486
Ending Balance, Shares at Jun. 30, 2021		142,359,774
Treasury Stock, Ending Balance, Shares at Jun. 30, 2021	(938,629)				(938,629)