Consolidated Statements of Shareholders' Equity (USD $) In Thousands, except Share data	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			(269,467)
NET EARNINGS (unaudited)	22,336			22,336
Stock option exercises (unaudited)	713		713
Stock option exercises, Shares (unaudited)		27,120
Cash dividends declared (unaudited)	(8,318)			(8,318)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	14,017						14,017
Additional tax benefit related to directors' deferred compensation plan (unaudited)	25		25
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					(128)	128
Shares purchased in connection with directors' deferred compensation plan, net, Shares (unaudited)					651
Stock option expense (unaudited)	146		146
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)		32,019,617			(268,816)
Ending Balance (unaudited) at Mar. 31, 2014	616,566	640	303,875	287,670	(5,618)	5,618	24,381
Ending Balance, Shares (unaudited) at Mar. 31, 2014		64,039,234			(537,632)
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			(529,563)
NET EARNINGS (unaudited)	23,997			23,997
Stock option exercises (unaudited)	728		728
Stock option exercises, Shares (unaudited)		52,891			0
Cash dividends declared (unaudited)	(8,980)			(8,980)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	8,761						8,761
Additional tax benefit related to directors' deferred compensation plan (unaudited)	25		25
Shares purchased in connection with directors' deferred compensation plan, net (unaudited)					(127)	127
Shares purchased in connection with directors' deferred compensation plan, net, Shares (unaudited)		0			2,030
Stock option expense (unaudited)	180		180
Ending Balance (unaudited) at Mar. 31, 2015	$ 706,248	$ 641	$ 306,362	$ 342,995	$ (6,005)	$ 6,005	$ 56,250
Ending Balance, Shares (unaudited) at Mar. 31, 2015		64,142,812			(527,533)