Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (USD $) In Thousands, unless otherwise specified	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	15,032			13,813	1,219
Cash dividends declared per share: $0.24 and $0.23 per share for the three months ended March 31, 2015 and March 31, 2014, respectively	(6,881)			(6,881)
Stock issued - stock options	557	27	530
Shares issued – directors’ stock plans	310	12	298
Shares issued – restricted stock units	(477)	36	(513)
Share-based compensation	554	1	553
Ending Balance at Mar. 31, 2014	705,595	29,866	489,045	205,985	(19,301)
Beginning Balance at Dec. 31, 2014	797,133	32,774	565,166	231,646	(32,453)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	20,509			17,835	2,674
Cash dividends declared per share: $0.24 and $0.23 per share for the three months ended March 31, 2015 and March 31, 2014, respectively	(7,899)			(7,899)
Stock issued - stock options	131	13	118
Shares issued – directors’ stock plans	316	11	305
Shares issued – restricted stock units	(444)	48	(492)
Share-based compensation	755	1	754
Ending Balance at Mar. 31, 2015	$ 810,501	$ 32,847	$ 565,851	$ 241,582	$ (29,779)