Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock Outstanding	Common Stock	Value of Shares Held in Rabbi Trust at Cost	Deferred Compensation and Other Retirement Benefit Obligations	Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance September 30, 2016 at Dec. 31, 2014		23,998,738
Balance September 30, 2015 at Dec. 31, 2014	$ 640,527		$ 237	$ (3,666)	$ 3,666	$ 311,978	$ 330,444	$ (2,132)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	45,505						45,505
Other comprehensive income	1,449							1,449
Common dividend declared ($0.87 and $0.78 per share for 2016 and 2015, respectively)	(20,412)						(20,412)
Common stock issued for acquisition		2,052,137
Common stock issued for acquisition	86,415		21			86,394
Proceeds from exercise of stock options, net of cash paid		78,240
Proceeds from exercise of stock options, net of cash paid	1,365		1			1,364
Tax benefit related to equity award activity	776					776
Stock based compensation	2,028					2,028
Restricted stock awards issued, net of awards surrendered		36,901
Restricted stock awards issued, net of awards surrendered	(645)		1			(646)
Shares issued under direct stock purchase plan		46,222
Shares issued under direct stock purchase plan	2,023					2,023
Deferred compensation and other retirement benefit obligations				(217)	217
Increase (Decrease) in Deferred Compensation	0
Tax benefit related to deferred compensation distributions	172					172
Balance December 31, 2015 at Sep. 30, 2015	759,203		260	(3,883)	3,883	404,089	355,537	(683)
Balance December 31, 2014 at Sep. 30, 2015		26,212,238
Balance September 30, 2016 at Dec. 31, 2015		26,236,352
Balance September 30, 2015 at Dec. 31, 2015	771,463		260	(3,958)	3,958	405,486	368,169	(2,452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	59,469						59,469
Other comprehensive income	5,952							5,952
Common dividend declared ($0.87 and $0.78 per share for 2016 and 2015, respectively)	(22,888)						(22,888)
Common stock issued for acquisition	0
Proceeds from exercise of stock options, net of cash paid		9,022
Proceeds from exercise of stock options, net of cash paid	160					160
Tax benefit related to equity award activity	354					354
Stock based compensation	2,308					2,308
Restricted stock awards issued, net of awards surrendered		32,567
Restricted stock awards issued, net of awards surrendered	(673)		1			(674)
Shares issued under direct stock purchase plan		42,526
Shares issued under direct stock purchase plan	1,918					1,918
Deferred compensation and other retirement benefit obligations				(241)	241
Tax benefit related to deferred compensation distributions	179					179
Balance December 31, 2015 at Sep. 30, 2016	$ 818,242		$ 261	$ (4,199)	$ 4,199	$ 409,731	$ 404,750	$ 3,500
Balance December 31, 2014 at Sep. 30, 2016		26,320,467