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	38,374			42,430	(4,056)
Cash dividends declared per share: $0.64 and $0.70 per share for the nine months ended September 30, 2013 and September 30, 2014, 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)
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: $0.64 and $0.70 per share for the nine months ended September 30, 2013 and September 30, 2014, respectively	(21,647)			(21,647)
Issuance of common stock, net of issuance costs	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)