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, 2016	$ 837,885	$ 661	$ 372,245	$ 446,534	$ (6,671)	$ 6,671	$ 18,445
Beginning Balance, Shares at Dec. 31, 2016		66,094,695
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2016					(507,409)
Net earnings (unaudited)	54,854			54,854
Stock option exercises (unaudited)	1,115	$ 1	1,114
Stock option exercises, Shares		60,967
Restricted stock grant (unaudited)	600		600
Restricted stock grant, Shares		14,650
Cash dividends declared (unaudited)	(24,476)			(24,476)
Change in unrealized gain (loss) in investment securities available-for-sale, net of related income taxes (unaudited)	17,032						17,032
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (277)	277
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					6,211
Stock option expense (unaudited)	432		432
Ending Balance at Jun. 30, 2017	$ 887,442	$ 662	374,391	476,912	$ (6,948)	6,948	35,477
Ending Balance, Shares at Jun. 30, 2017		66,170,312
Treasury Stock, Ending Balance, Shares at Jun. 30, 2017	(501,198)				(501,198)
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
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2017	(495,964)				(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, Shares		10,710
Cash dividends declared (unaudited)	(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 (loss) 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 (umaudited)				(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)