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)
Cash dividends declared per common share (in dollars per share)							$ 0.29
Balance March 31, 2017 at Dec. 31, 2015		26,236,352
Balance March 31, 2016 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	18,611						18,611
Other comprehensive income	4,264							4,264
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)	(7,627)						(7,627)
Proceeds from exercise of stock options, net of cash paid		5,000
Proceeds from exercise of stock options, net of cash paid	149		0			149
Tax benefit related to equity award activity	235					235
Stock based compensation	865					865
Restricted stock awards issued, net of awards surrendered		36,887
Restricted stock awards issued, net of awards surrendered	(671)		1			(672)
Shares issued under direct stock purchase plan		15,326
Shares issued under direct stock purchase plan	679					679
Deferred compensation and other retirement benefit obligations				(73)	73
Increase (Decrease) in Deferred Compensation	0
Tax benefit related to deferred compensation distributions	179					179
Balance December 31, 2016 at Mar. 31, 2016	788,147		261	(4,031)	4,031	406,921	$ 379,153	1,812
Balance December 31, 2015 at Mar. 31, 2016		26,293,565
Cash dividends declared per common share (in dollars per share)							$ 0.32
Balance March 31, 2017 at Dec. 31, 2016		27,005,813
Balance March 31, 2016 at Dec. 31, 2016	864,690		268	(4,277)	4,277	451,664	$ 414,095	(1,337)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,725						20,725
Other comprehensive income	698							698
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)	(8,653)						(8,653)
Proceeds from exercise of stock options, net of cash paid		7,688
Proceeds from exercise of stock options, net of cash paid	143					143
Stock based compensation	643					643
Restricted stock awards issued, net of awards surrendered		27,534
Restricted stock awards issued, net of awards surrendered	(1,336)		1			(1,337)
Shares issued under direct stock purchase plan		5,733
Shares issued under direct stock purchase plan	393					393
Deferred compensation and other retirement benefit obligations				(53)	53
Balance December 31, 2016 at Mar. 31, 2017	877,480		$ 269	$ (4,330)	$ 4,330	452,048	425,802	$ (639)
Balance December 31, 2015 at Mar. 31, 2017		27,046,768
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	$ 177					$ 542	$ (365)