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, 2017	$ 922,768	$ 663	$ 378,062	$ 517,257	$ (7,148)	$ 7,148	$ 26,786
Beginning Balance, Shares at Dec. 31, 2017		66,260,444			(495,964)
Net earnings (unaudited)	72,152			72,152
Stock option exercises (unaudited)	2,319	$ 1	2,318
Stock option exercises, Shares		109,133
Restricted stock grant (unaudited)	523		523
Restricted stock grant (unaudited), Shares		10,710
Cash dividends declared	(27,056)			(27,056)
Stock issued in acquisition of Commercial Bancshares, Inc.	58,087	$ 13	58,074
Stock issued in acquisition of Commercial Bancshares, Inc, Shares		1,289,371
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(43,919)						(43,919)
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (271)	271
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					4,794
Stock option expense (unaudited)	754		754
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				(5,759)			5,759
Reclassification of unrealized gain in equity securities at December 31, 2017 from accumulated other comprehensive earnings to retained earnings (unaudited)				21			(21)
Ending Balance at Jun. 30, 2018	$ 985,628	$ 677	439,731	556,615	$ (7,419)	7,419	(11,395)
Ending Balance, Shares at Jun. 30, 2018		67,669,658
Treasury Stock, Ending Balance, Shares at Jun. 30, 2018	(491,170)				(491,170)
Beginning Balance at Mar. 31, 2018	$ 969,814	$ 676	437,868	533,427	$ (7,291)	7,291	(2,157)
Beginning Balance, Shares at Mar. 31, 2018		67,612,760
Treasury Stock, Beginning Balance, Shares at Mar. 31, 2018					(493,994)
Net earnings (unaudited)	37,633			37,633
Stock option exercises (unaudited)	964	$ 1	963
Stock option exercises, Shares		46,188
Restricted stock grant (unaudited)	523		523
Restricted stock grant (unaudited), Shares		10,710
Cash dividends declared	(14,466)			(14,466)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(9,217)						(9,217)
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (128)	128
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					2,824
Stock option expense (unaudited)	377		377
Reclassification of unrealized gain in equity securities at December 31, 2017 from accumulated other comprehensive earnings to retained earnings (unaudited)				21			(21)
Ending Balance at Jun. 30, 2018	$ 985,628	$ 677	439,731	556,615	$ (7,419)	7,419	(11,395)
Ending Balance, Shares at Jun. 30, 2018		67,669,658
Treasury Stock, Ending Balance, Shares at Jun. 30, 2018	(491,170)				(491,170)
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)				(467,811)
Net earnings (unaudited)	$ 80,345			80,345
Stock option exercises (unaudited)	3,186	$ 2	3,184
Stock option exercises, Shares		168,349
Restricted stock grant (unaudited)	1,426		1,426
Restricted stock grant (unaudited), Shares		47,532
Cash dividends declared	(30,531)			(30,531)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	56,402						56,402
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (316)	316
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					2,709
Stock option expense (unaudited)	625		625
Two-for-one stock split in the form of a 100% stock dividend (unaudited)		$ 678		(678)
Two-for-one stock split in the form of a 100% stock dividend (unaudited), shares		67,840,210			(464,339)
Ending Balance at Jun. 30, 2019	$ 1,164,748	$ 1,358	448,349	655,794	$ (7,823)	7,823	59,247
Ending Balance, Shares at Jun. 30, 2019		135,809,224
Treasury Stock, Ending Balance, Shares at Jun. 30, 2019	(929,441)				(929,441)
Beginning Balance at Mar. 31, 2019	$ 1,107,522	$ 1,356	445,672	629,988	$ (7,660)	7,660	30,506
Beginning Balance, Shares at Mar. 31, 2019		135,680,420
Treasury Stock, Beginning Balance, Shares at Mar. 31, 2019					(928,678)
Net earnings (unaudited)	42,093			42,093
Stock option exercises (unaudited)	940	$ 2	938
Stock option exercises, Shares		81,272
Restricted stock grant (unaudited)	1,426		1,426
Restricted stock grant (unaudited), Shares		47,532
Cash dividends declared	(16,287)			(16,287)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	28,741						28,741
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)	0				$ (163)	163
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					(763)
Stock option expense (unaudited)	313		313
Ending Balance at Jun. 30, 2019	$ 1,164,748	$ 1,358	$ 448,349	$ 655,794	$ (7,823)	$ 7,823	$ 59,247
Ending Balance, Shares at Jun. 30, 2019		135,809,224
Treasury Stock, Ending Balance, Shares at Jun. 30, 2019	(929,441)				(929,441)