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, 2013	$ 587,647	$ 320	$ 302,991	$ 273,972	$ (5,490)	$ 5,490	$ 10,364
Beginning Balance, Shares at Dec. 31, 2013		31,992,497
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2013					(269,467)
NET EARNINGS (unaudited)	66,972			66,972
Stock option exercises (unaudited)	1,077	$ 1	1,076
Stock option exercises, Shares (unaudited)		50,571
Cash dividends declared (unaudited)	(26,261)			(26,261)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	28,531						28,531
Additional tax benefit related to directors' deferred compensation plan (unaudited)	267		267
Shares redeemed in connection with directors' deferred compensation plan, net (unaudited)					$ (307)	307
Shares redeemed in connection with directors' deferred compensation plan, net, Shares (unaudited)					5,345
Stock option expense (unaudited)	532		532
Two-for-one stock split in the form of a 100% stock dividend (unaudited)		$ 320		(320)
Two-for-one stock split in the form of a 100% stock dividend, Shares (unaudited)		32,022,760			(268,134)
Ending Balance (unaudited) at Sep. 30, 2014	$ 658,765	$ 641	304,866	314,363	$ (5,797)	5,797	38,895
Ending Balance, Shares (unaudited) at Sep. 30, 2014		64,065,828
Treasury Stock, Ending Balance, Shares (unaudited) at Sep. 30, 2014	(532,256)				(532,256)
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)				(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 (unaudited)		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 redeemed in connection with directors' deferred compensation plan, net (unaudited)					$ (329)	329
Shares redeemed in connection with directors' deferred compensation plan, net, Shares (unaudited)					5,400
Stock option expense (unaudited)	542		542
Ending Balance (unaudited) at Sep. 30, 2015	$ 792,034	$ 659	$ 367,444	$ 373,372	$ (6,207)	$ 6,207	$ 50,559
Ending Balance, Shares (unaudited) at Sep. 30, 2015		65,942,155
Treasury Stock, Ending Balance, Shares (unaudited) at Sep. 30, 2015	(524,163)				(524,163)