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)	37,232			37,232
Stock option exercises (unaudited)	2,192	$ 1	2,191
Stock option exercises, Shares		144,188
Restricted stock grant/fortfeiture, net (unaudited)	118		118
Restricted stock grant/fortfeiture, net (unaudited), Shares		3,413
Cash dividends declared	(17,060)			(17,060)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	56,070						56,070
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (215)	215
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					(1,009)
Stock option expense (unaudited)	340		340
Ending Balance at Mar. 31, 2020	$ 1,526,362	$ 1,423	673,535	727,828	$ (8,437)	8,437	123,576
Ending Balance, Shares at Mar. 31, 2020		142,314,930
Treasury Stock, Ending Balance, Shares at Mar. 31, 2020	(928,417)				(928,417)
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)	$ 56,918			56,918
Stock option exercises (unaudited)	1,904	$ 1	1,903
Stock option exercises, Shares		124,524
Restricted stock grant/fortfeiture, net (unaudited)	(17)		(17)
Restricted stock grant/fortfeiture, net (unaudited), Shares		(747)
Cash dividends declared	(18,500)			(18,500)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(53,386)						(53,386)
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (259)	259
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					587
Stock option expense (unaudited)	319		319
Ending Balance at Mar. 31, 2021	$ 1,665,428	$ 1,423	$ 671,849	$ 875,147	$ (9,385)	$ 9,385	$ 117,009
Ending Balance, Shares at Mar. 31, 2021		142,285,611
Treasury Stock, Ending Balance, Shares at Mar. 31, 2021	(938,004)				(938,004)