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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				27,450,190
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
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						$ (1)	$ 1
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Mar. 31, 2018		956,059			273	(4,591)	4,591	479,715	$ 484,266		(8,195)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Prior Period Reclassification Adjustment											(397)
Net income		58,673
Other comprehensive income		(7,055)
Cash dividends declared per common share (in dollars per share)									$ 0.76
Period End Balance at Jun. 30, 2018		977,065			274	(4,653)	4,653	481,979	$ 504,926		(10,114)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, Acquisitions		0
Period End Balance 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]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]	0							831		(831)
Common Stock, Shares, Outstanding				27,512,328
Net income		31,118							$ 31,118
Other comprehensive income		(1,919)									(1,919)
Cash dividends declared per common share (in dollars per share)									$ 0.38
Common dividend declared		(10,458)							$ (10,458)
Proceeds from exercise of stock options, net of cash paid				1,500
Proceeds from exercise of stock options, net of cash paid		41			0			41
Stock based compensation		1,351						1,351
Restricted stock awards issued, net of awards surrendered				6,256
Restricted stock awards issued, net of awards surrendered		(20)			1			(21)
Shares issued under direct stock purchase plan				12,440
Shares issued under direct stock purchase plan		893						893
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						(62)	62
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Jun. 30, 2018		977,065			274	(4,653)	4,653	481,979	504,926		(10,114)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				27,532,524
Common Stock, Shares, Outstanding				28,080,408
Net income		35,225							35,225
Other comprehensive income		8,054									8,054
Common dividend declared		(12,379)							(12,379)
Stock Issued During Period, Shares, Acquisitions				0
Stock Issued During Period, Value, Acquisitions		0			0			0
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
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						119	(119)
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Mar. 31, 2019		1,104,538			280	(4,599)	4,599	527,795	$ 569,582		6,881
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		65,853
Other comprehensive income		22,129
Cash dividends declared per common share (in dollars per share)									$ 0.88
Stock Issued During Period, Value, Acquisitions		499,693
Period End Balance at Jun. 30, 2019		1,636,003			342	(4,648)	4,648	1,029,594	$ 585,111		20,956
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				28,137,504
Net income		30,628							$ 30,628
Other comprehensive income		14,075									14,075
Cash dividends declared per common share (in dollars per share)									$ 0.44
Common dividend declared		(15,099)							$ (15,099)
Stock Issued During Period, Shares, Acquisitions				6,166,010
Stock Issued During Period, Value, Acquisitions		499,693			61			499,632
Proceeds from exercise of stock options, net of cash paid				5,000
Proceeds from exercise of stock options, net of cash paid		116						116
Stock based compensation		1,517						1,517
Restricted stock awards issued, net of awards surrendered				6,067
Restricted stock awards issued, net of awards surrendered		(12)			1			(13)
Shares issued under direct stock purchase plan				6,480
Shares issued under direct stock purchase plan		547						547
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						(49)	49
Period End Balance at Jun. 30, 2019		$ 1,636,003			$ 342	$ (4,648)	$ 4,648	$ 1,029,594	$ 585,111		$ 20,956
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				34,321,061

[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.