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)									$ 0.96
Balance September 30, 2018 at Dec. 31, 2016				27,005,813
Balance September 30, 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		65,140							65,140
Other comprehensive loss		1,753									1,753
Common dividend declared ($1.14 and $0.96 per share for 2018 and 2017, respectively)		(26,212)							(26,212)
Stock Issued During Period, Shares, Acquisitions				369,286
Proceeds from exercise of stock options, net of cash paid				11,590
Proceeds from exercise of stock options, net of cash paid					0
Stock Repurchased and Retired During Period, Value		(5)						(5)
Stock Issued During Period, Value, Acquisitions		23,468			4			23,464
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
Increase (Decrease) in Deferred Compensation		0
Balance December 31, 2017 at Sep. 30, 2017		931,224			273	(4,505)	4,505	477,877	452,658		416
Balance December 31, 2016 at Sep. 30, 2017				27,437,791
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)									$ 1.14
Balance September 30, 2018 at Dec. 31, 2017				27,450,190
Balance September 30, 2017 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	[2]						397	[3]	(397)	[3]
Net income		91,688							91,688
Other comprehensive loss		(9,605)									(9,605)
Common dividend declared ($1.14 and $0.96 per share for 2018 and 2017, respectively)		(31,380)							(31,380)
Proceeds from exercise of stock options, net of cash paid				23,195
Proceeds from exercise of stock options, net of cash paid		184						184
Stock Issued During Period, Value, Acquisitions		0
Stock based compensation		3,161						3,161
Restricted stock awards issued, net of awards surrendered				43,174
Restricted stock awards issued, net of awards surrendered		(1,344)			1			(1,345)
Shares issued under direct stock purchase plan				24,284
Shares issued under direct stock purchase plan		1,792						1,792
Deferred compensation and other retirement benefit obligations						(194)	194
Balance December 31, 2017 at Sep. 30, 2018		998,305			$ 274	$ (4,784)	$ 4,784	483,222	527,473		(12,664)
Balance December 31, 2016 at Sep. 30, 2018				27,540,843
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	[3]					$ 0	$ 831	[2]	$ (831)	[2]

[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 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.

[3] 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.