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, 2014	$ 681,537	$ 641	$ 305,429	$ 327,978	$ (5,878)	$ 5,878	$ 47,489
Beginning Balance, Shares at Dec. 31, 2014		64,089,921
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2014					(529,563)
Net earnings (unaudited)	75,189			75,189
Stock option exercises (unaudited)	1,290		1,290
Stock option exercises, Shares (Unaudited)		89,790
Restricted stock grant (unaudited)	250		250
Restricted stock grant, Shares (unaudited)		7,070
Stock issued in acquisition of FBC Bancshares, Inc. (unaudited)	59,648	$ 18	59,630
Stock issued in acquisition of FBC Bancshares, Inc., Shares		1,755,374
Cash dividends declared (Unaudited)	(29,795)			(29,795)
Minimum liability pension adjustment, net of related income taxes (unaudited)	(140)						(140)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	3,210						3,210
Additional tax benefit related to directors' deferred compensation plan (unaudited)	303		303
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (329)	329
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)					5,400
Stock option expense (unaudited)	542		542
Ending Balance at Sep. 30, 2015	$ 792,034	$ 659	367,444	373,372	$ (6,207)	6,207	50,559
Ending Balance, Shares at Sep. 30, 2015		65,942,155
Treasury Stock, Ending Balance, Shares at Sep. 30, 2015	(524,163)				(524,163)
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)				(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)	0				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)