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 [Member]
Beginning Balance at Dec. 31, 2015	$ 804,986	$ 660	$ 368,925	$ 388,006	$ (6,296)	$ 6,296	$ 47,395
Beginning Balance, Shares at Dec. 31, 2015		65,990,234
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2015					(520,651)
Net earnings (unaudited)	78,108			78,108
Stock option exercises (unaudited)	989	$ 1	988
Stock option exercises, Shares (Unaudited)		66,866
Restricted stock grant (unaudited)	250		250
Restricted stock grant, Shares (unaudited)		6,185
Cash dividends declared (Unaudited)	(34,349)			(34,349)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	16,953						16,953
Additional tax benefit related to directors' deferred compensation plan (unaudited)	345		345
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (270)	270
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)					9,696
Stock option expense (unaudited)	662		662
Ending Balance at Sep. 30, 2016	$ 867,944	$ 661	371,170	431,765	$ (6,566)	6,566	64,348
Ending Balance, Shares at Sep. 30, 2016		66,063,285
Treasury Stock, Ending Balance, Shares at Sep. 30, 2016	(510,955)				(510,955)
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)				(507,409)
Net earnings (unaudited)	$ 84,227			84,227
Stock option exercises (unaudited)	2,260	$ 1	2,259
Stock option exercises, Shares (Unaudited)		114,612
Restricted stock grant (unaudited)	600		600
Restricted stock grant, Shares (unaudited)		14,650
Cash dividends declared (Unaudited)	(37,055)			(37,055)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	17,457						17,457
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (357)	357
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)					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)