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)	25,696			25,696
Stock option exercises (unaudited)	802		802
Stock option exercises, Shares (Unaudited)		54,683
Restricted stock grant (unaudited)	0	$ 0	0	0	$ 0	0	0
Restricted stock grant, Shares (unaudited)		1,475
Cash dividends declared (Unaudited)	(10,567)			(10,567)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	17,446						17,446
Additional tax benefit related to directors' deferred compensation plan (unaudited)	25		25
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (113)	113
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)					294
Stock option expense (unaudited)	220		220
Ending Balance at Mar. 31, 2016	$ 838,608	$ 660	369,972	403,135	$ (6,409)	6,409	64,841
Ending Balance, Shares at Mar. 31, 2016		66,043,442
Treasury Stock, Ending Balance, Shares at Mar. 31, 2016	(520,357)				(520,357)
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)	$ 26,599			26,599
Stock option exercises (unaudited)	563		563
Stock option exercises, Shares (Unaudited)		37,137
Restricted stock grant (unaudited)	0	$ 0	0	0	$ 0	0	0
Cash dividends declared (Unaudited)	(11,904)			(11,904)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	$ 5,996						5,996
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					$ (119)	119
Shares purchased in connection with directors' deferred compensation plan, net, Shares (Unaudited)	0				4,304
Stock option expense (unaudited)	$ 215		215
Ending Balance at Mar. 31, 2017	$ 859,354	$ 661	$ 373,023	$ 461,229	$ (6,790)	$ 6,790	$ 24,441
Ending Balance, Shares at Mar. 31, 2017		66,131,832
Treasury Stock, Ending Balance, Shares at Mar. 31, 2017	(503,105)				(503,105)