Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2013	$ 696,500	$ 29,790	$ 488,177	$ 199,053	$ (20,520)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	47,830			46,816	1,014
Cash dividends declared per share: $0.74 and $0.70 per share for the nine months ended September 30, 2015 and September 30, 2014, respectively	(21,647)			(21,647)
Stock Issued During Period, Value, New Issues	76,175	2,875	73,300
Stock issued - stock options	901	43	858
Shares issued – directors’ stock plans	468	18	450
Shares issued – restricted stock units	(502)	$ 37	(539)
Share-based compensation	1,881		1,881
Ending Balance at Sep. 30, 2014	801,606	$ 32,763	564,127	224,222	(19,506)
Beginning Balance at Dec. 31, 2014	797,133	32,774	565,166	231,646	(32,453)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	64,593			61,326	3,267
Cash dividends declared per share: $0.74 and $0.70 per share for the nine months ended September 30, 2015 and September 30, 2014, respectively	(26,981)			(26,981)
Stock Issued During Period, Value, Acquisitions	159,904	5,183	154,721
Stock issued - stock options	1,234	98	1,136
Shares issued – directors’ stock plans	316	11	305
Shares issued – restricted stock units	(343)	54	(397)
Share-based compensation	2,507	11	2,496
Ending Balance at Sep. 30, 2015	$ 998,363	$ 38,131	$ 723,427	$ 265,991	$ (29,186)