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	23,491			27,437	(3,946)
Cash dividends declared per share: $0.42 and $0.46 per share for the six months ended June 30, 2013 and June 30, 2014, respectively	(11,584)			(11,584)
Stock issued - stock options	16	3	13
Shares issued – directors’ stock plans	305	13	292
Shares issued – restricted stock units	(379)	23	(402)
Share-based compensation	1,381		1,381
Ending Balance at Jun. 30, 2013	609,571	27,538	434,479	182,619	(35,065)
Beginning Balance at Dec. 31, 2013	696,500	29,790	488,177	199,053	(20,520)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	32,581			30,049	2,532
Cash dividends declared per share: $0.42 and $0.46 per share for the six months ended June 30, 2013 and June 30, 2014, respectively	(13,769)			(13,769)
Issuance of common stock, net of issuance costs	76,175	2,875	73,300
Stock issued - stock options	854	41	813
Shares issued – directors’ stock plans	468	18	450
Shares issued – restricted stock units	(484)	36	(520)
Share-based compensation	1,173		1,173
Ending Balance at Jun. 30, 2014	$ 793,498	$ 32,760	$ 563,393	$ 215,333	$ (17,988)