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)							$ 0.58
Balance June 30, 2017 at Dec. 31, 2015		26,236,352
Balance June 30, 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	38,985						38,985
Other comprehensive income	6,035							6,035
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)	(15,256)						(15,256)
Proceeds from exercise of stock options, net of cash paid		6,652
Proceeds from exercise of stock options, net of cash paid	144		0			144
Tax benefit related to equity award activity	327					327
Stock based compensation	1,633					1,633
Restricted stock awards issued, net of awards surrendered		42,967
Restricted stock awards issued, net of awards surrendered	(673)		1			(674)
Shares issued under direct stock purchase plan		23,916
Shares issued under direct stock purchase plan	1,060					1,060
Deferred compensation and other retirement benefit obligations				(155)	155
Increase (Decrease) in Deferred Compensation	0
Tax benefit related to deferred compensation distributions	179					179
Balance December 31, 2016 at Jun. 30, 2016	803,897		261	(4,113)	4,113	408,155	$ 391,898	3,583
Balance December 31, 2015 at Jun. 30, 2016		26,309,887
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, Acquisitions	0
Cash dividends declared per common share (in dollars per share)							$ 0.64
Balance June 30, 2017 at Dec. 31, 2016		27,005,813
Balance June 30, 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	41,288						41,288
Other comprehensive income	1,378							1,378
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)	(17,431)						(17,431)
Proceeds from exercise of stock options, net of cash paid		11,174
Proceeds from exercise of stock options, net of cash paid	8					8
Stock based compensation	1,560					1,560
Restricted stock awards issued, net of awards surrendered		32,524
Restricted stock awards issued, net of awards surrendered	(1,361)					(1,361)
Shares issued under direct stock purchase plan		12,374
Shares issued under direct stock purchase plan	807					807
Deferred compensation and other retirement benefit obligations				(137)	137
Balance December 31, 2016 at Jun. 30, 2017	914,584		272	$ (4,414)	$ 4,414	476,684	437,587	$ 41
Balance December 31, 2015 at Jun. 30, 2017		27,431,171
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, Acquisitions		369,286
Stock Issued During Period, Value, Acquisitions	23,468		$ 4			23,464
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	$ 177					$ 542	$ (365)