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)								$ 0.38
Balance March 31, 2019 at Dec. 31, 2017			27,450,190
Balance March 31, 2018 at Dec. 31, 2017	$ 943,809			$ 273	$ (4,590)	$ 4,590	$ 479,430	$ 465,937		$ (1,831)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Prior Period Reclassification Adjustment	0	[1]						397	[2]	(397)	[2]
Net income	27,555							27,555
Other comprehensive income	(5,136)									(5,136)
Common dividend declared	(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			0			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)			0			(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
Increase (Decrease) in Deferred Compensation	0
Balance December 31, 2018 at Mar. 31, 2018	956,059			273	(4,591)	4,591	479,715	484,266		(8,195)
Balance December 31, 2017 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	[2]						$ 831	[1]	(831)	[1]
Cash dividends declared per common share (in dollars per share)								$ 0.44
Balance March 31, 2019 at Dec. 31, 2018			28,080,408
Balance March 31, 2018 at Dec. 31, 2018	1,073,490			279	(4,718)	4,718	527,648	$ 546,736		(1,173)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	35,225							35,225
Other comprehensive income	8,054									8,054
Common dividend declared	(12,379)							(12,379)
Proceeds from exercise of stock options, net of cash paid			6,000
Proceeds from exercise of stock options, net of cash paid	165						165
Stock based compensation	915						915
Restricted stock awards issued, net of awards surrendered			44,407
Restricted stock awards issued, net of awards surrendered	(1,419)			1			(1,420)
Shares issued under direct stock purchase plan			6,689
Shares issued under direct stock purchase plan	487						487
Deferred compensation and other retirement benefit obligations					119	(119)
Balance December 31, 2018 at Mar. 31, 2019	$ 1,104,538			$ 280	$ (4,599)	$ 4,599	$ 527,795	$ 569,582		$ 6,881
Balance December 31, 2017 at Mar. 31, 2019			28,137,504

[1] 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.

[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.