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 (loss)	41,769			39,346	2,423
Cash dividends declared per share: $0.61 and $0.64 per share for the nine months ended September 30, 2012 and September 30, 2013, respectively	(16,786)			(16,786)
Stock issued - stock options	11	1	10
Shares issued – directors’ stock plans	323	16	307
Shares issued – restricted stock units	(248)	24	(272)
Share-based compensation	1,305		1,305
Ending Balance at Sep. 30, 2012	598,103	27,498	432,627	160,884	(22,906)
Beginning Balance at Dec. 31, 2012	596,341	27,499	433,195	166,766	(31,119)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	38,374			42,430	(4,056)
Cash dividends declared per share: $0.61 and $0.64 per share for the nine months ended September 30, 2012 and September 30, 2013, respectively	(17,658)			(17,658)
Issuance of common stock, net of issuance costs	53,925	2,214	51,711
Stock issued - stock options	278	29	249
Shares issued – directors’ stock plans	305	13	292
Shares issued – restricted stock units	(379)	23	(402)
Share-based compensation	2,131		2,131
Ending Balance at Sep. 30, 2013	$ 673,317	$ 29,778	$ 487,176	$ 191,538	$ (35,175)