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)	52,505			52,505
Stock option exercises (unaudited)	936	$ 1	935
Stock option exercises, Shares (Unaudited)		62,018
Restricted stock grant (unaudited)	250		250
Restricted stock grant, Shares (unaudited)		7,660
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)					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)
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)	$ 54,854			54,854
Stock option exercises (unaudited)	1,115	$ 1	1,114
Stock option exercises, Shares (Unaudited)		60,967
Restricted stock grant (unaudited)	600		600
Restricted stock grant, Shares (unaudited)		14,650
Cash dividends declared (Unaudited)	(24,476)			(24,476)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	17,032						17,032
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (277)	277
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)					6,211
Stock option expense (unaudited)	432		432
Ending Balance at Jun. 30, 2017	$ 887,442	$ 662	$ 374,391	$ 476,912	$ (6,948)	$ 6,948	$ 35,477
Ending Balance, Shares at Jun. 30, 2017		66,170,312
Treasury Stock, Ending Balance, Shares at Jun. 30, 2017	(501,198)				(501,198)