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, 2018	$ 1,053,295	$ 678	$ 443,114	$ 606,658	$ (7,507)	$ 7,507	$ 2,845
Beginning Balance, Shares at Dec. 31, 2018		67,753,133
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2018					(467,811)
Net earnings (unaudited)	38,252			38,252
Stock option exercises (unaudited)	2,246	$ 0	2,246
Stock option exercises, Shares		87,077
Cash dividends declared	(14,244)			(14,244)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	27,661						27,661
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (153)	153
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					3,472
Stock option expense (unaudited)	312		312
Two-for-one stock spllit in the form of a 100% stock dividend (unaudited)		$ 678		(678)	$ (464,339)
Two-for-one stock spllit in the form of a 100% stock dividend (unaudited), shares		67,840,210
Ending Balance at Mar. 31, 2019	$ 1,107,522	$ 1,356	445,672	629,988	$ (7,660)	7,660	30,506
Ending Balance, Shares at Mar. 31, 2019		135,680,420
Treasury Stock, Ending Balance, Shares at Mar. 31, 2019	(928,678)				(928,678)
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
Stock issued in acquisition of Commercial Bancshares, Inc.	$ 220,273	$ 63	220,210
Stock issued in acquisition of Commercial Bancshares, Inc, Shares		6,275,574
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2019	(927,408)				(927,408)
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 (unaudited)	118		118
Restricted stock grant (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)