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)	84,227			84,227
Stock option exercises (unaudited)	2,260	$ 1	2,259
Stock option exercises, Shares		114,612
Restricted stock grant (unaudited)	600		600
Restricted stock grant, Shares		14,650
Cash dividends declared (unaudited)	(37,055)			(37,055)
Change in unrealized gain (loss) in investment securities available-for-sale, net of related income taxes (unaudited)	17,457						17,457
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (357)	357
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					8,950
Stock option expense (unaudited)	1,182		1,182
Ending Balance at Sep. 30, 2017	$ 906,556	$ 662	376,286	493,706	$ (7,028)	7,028	35,902
Ending Balance, Shares at Sep. 30, 2017		66,223,957
Treasury Stock, Ending Balance, Shares at Sep. 30, 2017	(498,459)				(498,459)
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)	$ 112,202			112,202
Stock option exercises (unaudited)	2,800	$ 1	2,799
Stock option exercises, Shares		133,061
Restricted stock grant (unaudited)	523		523
Restricted stock grant, Shares		10,710
Cash dividends declared (unaudited)	(41,272)			(41,272)
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)	(60,500)						(60,500)
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (364)	364
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					24,894
Stock option expense (unaudited)	1,131		1,131
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 (unaudited)				(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 Sep. 30, 2018	$ 995,739	$ 677	$ 440,589	$ 582,449	$ (7,512)	$ 7,512	$ (27,976)
Ending Balance, Shares at Sep. 30, 2018		67,693,586
Treasury Stock, Ending Balance, Shares at Sep. 30, 2018	(471,070)				(471,070)