Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2014	$ 797,133	$ 32,774	$ 565,166	$ 231,646	$ (32,453)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	20,509			17,835	2,674
Cash dividends declared and paid, $0.26 and $0.24 per share at March 31, 2016 and March 31, 2015, respectively.	7,899			7,899
Stock issued - stock options	131	13	118
Shares issued – directors’ stock plans	316	11	305
Shares issued – restricted stock units	(444)	48	(492)
Share-based compensation	755	1	754
Ending Balance at Mar. 31, 2015	810,501	32,847	565,851	241,582	(29,779)
Beginning Balance at Dec. 31, 2015	1,015,974	38,168	725,280	281,558	(29,032)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	25,604			23,262	2,342
Cash dividends declared and paid, $0.26 and $0.24 per share at March 31, 2016 and March 31, 2015, respectively.	9,961			9,961
Stock issued - stock options	382	36	346
Shares issued – restricted stock units	(457)	43	(500)
Share-based compensation	749	1	748
Ending Balance at Mar. 31, 2016	$ 1,032,291	$ 38,248	$ 725,874	$ 294,859	$ (26,690)