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 Loss
Cash dividends declared per common share (in dollars per share)								$ 0.32
Balance March 31, 2018 at Dec. 31, 2016			27,005,813
Balance March 31, 2017 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 loss		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		0			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
Increase (Decrease) in Deferred Compensation		0
Balance December 31, 2017 at Mar. 31, 2017		877,480		269	(4,330)	4,330	452,048	$ 425,802		(639)
Balance December 31, 2016 at Mar. 31, 2017			27,046,768
Cash dividends declared per common share (in dollars per share)								$ 0
Balance March 31, 2018 at Dec. 31, 2016			27,005,813
Balance March 31, 2017 at Dec. 31, 2016		864,690		268	(4,277)	4,277	451,664	$ 414,095		(1,337)
Balance December 31, 2017 at Dec. 31, 2017		943,809		273	(4,590)	4,590	479,430	465,937		(1,831)
Balance December 31, 2016 at Dec. 31, 2017			27,450,190
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]	177					542	$ (365)
Cash dividends declared per common share (in dollars per share)								$ 0.38
Prior Period Reclassification Adjustment		0						$ 397	[2]	(397)	[2]
Net income		27,555						27,555
Other comprehensive loss		(5,136)								(5,136)
Common dividend declared ($0.32 and $0.29 per share for 2017 and 2016, respectively)		(10,454)						(10,454)
Proceeds from exercise of stock options, net of cash paid			19,256
Proceeds from exercise of stock options, net of cash paid		143					143
Stock based compensation		1,041					1,041
Restricted stock awards issued, net of awards surrendered			36,961
Restricted stock awards issued, net of awards surrendered		(1,318)					(1,318)
Shares issued under direct stock purchase plan			5,921
Shares issued under direct stock purchase plan		419					419
Deferred compensation and other retirement benefit obligations					(1)	1
Balance December 31, 2017 at Mar. 31, 2018		956,059		$ 273	$ (4,591)	$ 4,591	$ 479,715	484,266		(8,195)
Balance December 31, 2016 at Mar. 31, 2018			27,512,328
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets		$ 0						$ 831	[3]	$ (831)	[3]

[1] Represents adjustment needed to reflect the cumulative impact on retained earnings for previously recognized stock based compensation, which included an adjustment for estimated forfeitures. Pursuant to the Company's adoption of Accounting Standards Update 2016-09, the Company has elected to recognize stock based compensation without inclusion of a forfeiture estimate, and as such has recognized this adjustment to present retained earnings consistent with this election.

[2] Represents adjustment needed to reflect the cumulative impact on retained earnings for reclassification of the income tax effects attributable to accumulated other comprehensive income, as a result of the Tax Cuts and Jobs Act (the "Tax Act"). Pursuant to the Company's adoption of Accounting Standards Update 2018-02, the Company has elected to reclassify amounts stranded in other comprehensive income to retained earnings.

[3] Represents adjustment needed to reflect the cumulative impact on retained earnings for the classification and measurement of investments in equity securities. Pursuant to the Company's adoption of Accounting Standards Update 2016-01, the Company's investments in equity securities will no longer be classified as available for sale, therefore the Company was required to reclassify the net unrealized gain recognized on the change in fair value of these equity securities from other comprehensive income to retained earnings.