CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock Outstanding [Member]	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance at at Dec. 31, 2014	$ 521,751		$ 25,045	$ 194,647	$ 302,882	$ (823)
Net income	45,355		0	0	45,355	0
Other comprehensive income, net of tax	(474)		0	0	0	(474)
Common stock dividends - $1.04, $.98 and $0.90 per share at December 31, 2017, 2016 and 2015 respectively.	(22,397)		0	0	(22,397)	0
Stock compensation expense	1,979		0	1,979	0	0
Common stock issued pursuant to:
Stock option plan - 30,567, 44067, 39,787 shares at December 31, 2017, 2016 and 2015, respectively	602		40	562	0	0
Director's stock purchase plan - 258 and 837 shares at December 31, 2016 and 2015 respectively	22		1	21	0	0
Employee stock purchase plan - 17,578, 23,779, 25,136 shares at December 31, 2017, 2016, 2015 respectively	566		25	541	0	0
Restricted stock - 47,064 , 49,648, 55,874 shares at December 31, 2017, 2016 and 2015, respectively	(353)		56	(409)	0	0
Purchase of treasury shares - 512,459 , 870,450 shares at December 31, 2016, 2015	(22,624)	$ (871)		(21,753)	0	0
Balance at at Dec. 31, 2015	524,427		24,296	175,588	325,840	(1,297)
Net income	48,250		0	0	48,250	0
Other comprehensive income, net of tax	(5,317)		0	0	0	(5,317)
Common stock dividends - $1.04, $.98 and $0.90 per share at December 31, 2017, 2016 and 2015 respectively.	(23,676)		0	0	(23,676)	0
Stock compensation expense	2,264		0	2,264	0	0
Common stock issued pursuant to:
Stock option plan - 30,567, 44067, 39,787 shares at December 31, 2017, 2016 and 2015, respectively	716		44	672	0	0
Director's stock purchase plan - 258 and 837 shares at December 31, 2016 and 2015 respectively	7		0	7	0	0
Employee stock purchase plan - 17,578, 23,779, 25,136 shares at December 31, 2017, 2016, 2015 respectively	591		24	567	0	0
Restricted stock - 47,064 , 49,648, 55,874 shares at December 31, 2017, 2016 and 2015, respectively	(417)		49	(466)	0	0
Purchase of treasury shares - 512,459 , 870,450 shares at December 31, 2016, 2015	(13,273)	$ (512)		(12,761)	0	0
Balance at at Dec. 31, 2016	533,572		23,901	165,871	350,414	(6,614)
Net income	53,209		0	0	53,209	0
Other comprehensive income, net of tax	(243)		0	0	0	(243)
Common stock dividends - $1.04, $.98 and $0.90 per share at December 31, 2017, 2016 and 2015 respectively.	(25,134)		0	0	(25,134)	0
Stock compensation expense	2,164		0	2,164	0	0
Common stock issued pursuant to:
Stock option plan - 30,567, 44067, 39,787 shares at December 31, 2017, 2016 and 2015, respectively	593		31	562	0	0
Employee stock purchase plan - 17,578, 23,779, 25,136 shares at December 31, 2017, 2016, 2015 respectively	607		17	590	0	0
Restricted stock - 47,064 , 49,648, 55,874 shares at December 31, 2017, 2016 and 2015, respectively	(952)		47	(999)	0	0
Balance at at Dec. 31, 2017	$ 563,816		$ 23,996	$ 168,188	$ 378,489	$ (6,857)