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)	49,303			49,303
Stock option exercises (unaudited)	925	$ 1	924
Stock option exercises, Shares (Unaudited)		66,381
Cash dividends declared (Unaudited)	(19,244)			(19,244)
Minimum liability pension adjustment, net of related income taxes (unaudited)	(70)						(70)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	(11,668)						(11,668)
Additional tax benefit related to directors' deferred compensation plan (unaudited)	50		50
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (232)	232
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)					2,562
Stock option expense (unaudited)	360		360
Ending Balance at Jun. 30, 2015	$ 701,193	$ 642	306,763	358,037	$ (6,110)	6,110	35,751
Ending Balance, Shares at Jun. 30, 2015		64,156,302
Treasury Stock, Ending Balance, Shares at Jun. 30, 2015	(527,001)				(527,001)
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)	$ 52,505			52,505
Stock option exercises (unaudited)	936	$ 1	935
Stock option exercises, Shares (Unaudited)		63,493
Restricted stock grant (unaudited)	250		250
Restricted stock grant, Shares (Unaudited)		6,185
Cash dividends declared (Unaudited)	(22,458)			(22,458)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	29,448						29,448
Additional tax benefit related to directors' deferred compensation plan (unaudited)	$ 50		50
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (221)	221
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)	0				3,696
Stock option expense (unaudited)	$ 441		441
Ending Balance at Jun. 30, 2016	$ 866,158	$ 661	$ 370,601	$ 418,053	$ (6,517)	$ 6,517	$ 76,843
Ending Balance, Shares at Jun. 30, 2016		66,059,912
Treasury Stock, Ending Balance, Shares at Jun. 30, 2016	(516,955)				(516,955)