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, 2011	$ 571,729	$ 27,457	$ 431,277	$ 138,324	$ (25,329)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	13,592			12,374	1,218
Cash dividends declared per share: $0.20 and $0.21 per share for three months ended March 31, 2012 and March 31, 2013, respectively	(5,503)			(5,503)
Shares issued – directors’ stock plans	240	12	228
Shares issued – restricted stock units	(248)	22	(270)
Share-based compensation	314		314
Ending Balance at Mar. 31, 2012	580,124	27,491	431,549	145,195	(24,111)
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: $0.20 and $0.21 per share for three months ended March 31, 2012 and March 31, 2013, 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)