Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2010	$ 560,078	$ 27,440	$ 429,511	$ 117,238	$ (14,111)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	31,832			43,050	(11,218)
Cash dividends declared per share: $0.80 per share in 2011, $0.82 per share in 2012 and $0.87 per share in 2013	(21,964)			(21,964)
Shares issued - directors' stock plans	266	12	254
Share-based compensation	1,517	5	1,512
Ending Balance at Dec. 31, 2011	571,729	27,457	431,277	138,324	(25,329)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	45,218			51,008	(5,790)
Cash dividends declared per share: $0.80 per share in 2011, $0.82 per share in 2012 and $0.87 per share in 2013	(22,566)			(22,566)
Stock issued - stock options	20	1	19
Shares issued - directors' stock plans	323	16	307
Shares issued — restricted stock performance units	(248)	25	(273)
Share-based compensation	1,865		1,865
Ending Balance at Dec. 31, 2012	596,341	27,499	433,195	166,766	(31,119)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	67,407			56,808	10,599
Cash dividends declared per share: $0.80 per share in 2011, $0.82 per share in 2012 and $0.87 per share in 2013	(24,521)			(24,521)
Stock Issued During Period, Value, New Issues	53,925	2,214	51,711
Stock issued - stock options	568	41	527
Shares issued - directors' stock plans	305	13	292
Shares issued — restricted stock performance units	(379)	23	(402)
Share-based compensation	2,854		2,854
Ending Balance at Dec. 31, 2013	$ 696,500	$ 29,790	$ 488,177	$ 199,053	$ (20,520)