Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	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	67,407			56,808	10,599
Cash dividends declared per share: $0.87 per share in 2013, $0.94 per share in 2014 and $1.00 per share in 2015	(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)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	50,188			62,121	(11,933)
Cash dividends declared per share: $0.87 per share in 2013, $0.94 per share in 2014 and $1.00 per share in 2015	(29,528)			(29,528)
Stock Issued During Period, Value, New Issues	76,175	2,875	73,300
Stock issued - stock options	931	44	887
Shares issued - directors' stock plans	438	18	420
Shares issued — restricted stock performance units	(502)	37	(539)
Share-based compensation	2,931	10	2,921
Ending Balance at Dec. 31, 2014	797,133	32,774	565,166	231,646	(32,453)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	90,251			86,830	3,421
Cash dividends declared per share: $0.87 per share in 2013, $0.94 per share in 2014 and $1.00 per share in 2015	(36,918)			(36,918)
Stock Issued During Period, Value, Acquisitions	159,904	5,183	154,721
Stock issued - stock options	2,157	135	2,022
Shares issued - directors' stock plans	316	11	305
Shares issued — restricted stock performance units	(347)	54	(401)
Share-based compensation	3,478	11	3,467
Ending Balance at Dec. 31, 2015	$ 1,015,974	$ 38,168	$ 725,280	$ 281,558	$ (29,032)