Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Deferred Benefits for Directors [Member]
Beginning Balance at Dec. 31, 2016	$ 1,341,408	$ 91,524	$ 680,507	$ 597,071		$ (27,126)	$ (568)
Beginning Balance, shares at Dec. 31, 2016		43,931,715
Net income	52,228			52,228
Other comprehensive income	5,008					5,008
Comprehensive income	57,236
Common dividends declared	(22,878)			(22,878)
Treasury shares acquired	(488)				$ (488)
Treasury shares acquired, shares		(12,987)
Stock options exercised	1,061	$ 75	883		103
Stock options exercised, shares		38,584
Restricted stock granted		$ 154	(154)
Restricted stock granted, shares		74,023
Stock compensation expense	1,198		1,198
Deferred benefits for directors - net			9				(9)
Ending Balance at Jun. 30, 2017	1,377,537	$ 91,753	682,443	626,421	(385)	(22,118)	(577)
Ending Balance, shares at Jun. 30, 2017		44,031,335
Beginning Balance at Dec. 31, 2017	1,395,321	$ 91,756	684,730	651,357		(31,495)	(1,027)
Beginning Balance, shares at Dec. 31, 2017		44,043,244
Net income	66,698			66,698
Other comprehensive income	(20,794)					(20,794)
Comprehensive income	45,904
Common dividends declared	(26,298)			(26,298)
Adoption of accounting standard ASU 2016-01				1,063		(1,063)
Shares issued for acquisition	107,347	$ 5,206	102,141
Shares issued for acquisition, shares		2,498,761
Treasury shares acquired	(680)		34		(714)
Treasury shares acquired, shares		(15,159)
Stock options exercised	1,143	$ 69	915		159
Stock options exercised, shares		36,788
Restricted stock granted		$ 166	(166)
Restricted stock granted, shares		79,616
Stock compensation expense	1,838		1,838
Deferred benefits for directors - net	(469)		(454)				(15)
Ending Balance at Jun. 30, 2018	$ 1,524,106	$ 97,197	$ 789,038	$ 692,820	$ (555)	$ (53,352)	$ (1,042)
Ending Balance, shares at Jun. 30, 2018		46,643,250