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, 2012	$ 596,341	$ 27,499	$ 433,195	$ 166,766	$ (31,119)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	13,719			13,234	485
Cash dividends declared per share: $0.21 and $0.23 per share for the three months ended March 31, 2013 and March 31, 2014, respectively	(5,791)			(5,791)
Stock issued - stock options	(10)		(10)
Shares issued – directors’ stock plans	239	11	228
Shares issued – restricted stock units	(372)	22	(394)
Share-based compensation	629		629
Ending Balance at Mar. 31, 2013	604,755	27,532	433,648	174,209	(30,634)
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: $0.21 and $0.23 per share for the three months ended March 31, 2013 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)