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)							$ 0.87
Balance September 30, 2017 at Dec. 31, 2015		26,236,352
Balance September 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	59,469						59,469
Other comprehensive income	5,952							5,952
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)	(22,888)						(22,888)
Proceeds from exercise of stock options, net of cash paid		9,022
Proceeds from exercise of stock options, net of cash paid	160		0			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
Increase (Decrease) in Deferred Compensation	0
Tax benefit related to deferred compensation distributions	179					179
Balance December 31, 2016 at Sep. 30, 2016	818,242		261	(4,199)	4,199	409,731	$ 404,750	3,500
Balance December 31, 2015 at Sep. 30, 2016		26,320,467
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, Acquisitions	0
Cash dividends declared per common share (in dollars per share)							$ 0.96
Balance September 30, 2017 at Dec. 31, 2016		27,005,813
Balance September 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	65,140						65,140
Other comprehensive income	1,753							1,753
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)	(26,212)						(26,212)
Proceeds from exercise of stock options, net of cash paid		11,590
Stock Repurchased and Retired During Period, Value	(5)					(5)
Stock based compensation	2,346					2,346
Restricted stock awards issued, net of awards surrendered		32,476
Restricted stock awards issued, net of awards surrendered	(1,363)		1			(1,364)
Shares issued under direct stock purchase plan		18,626
Shares issued under direct stock purchase plan	1,230					1,230
Deferred compensation and other retirement benefit obligations				(228)	228
Balance December 31, 2016 at Sep. 30, 2017	931,224		273	$ (4,505)	$ 4,505	477,877	452,658	$ 416
Balance December 31, 2015 at Sep. 30, 2017		27,437,791
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)