Consolidated Statements of Changes In Stockholders' Equity - USD ($) $ in Thousands		Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Unallocated ESOP Shares	Accumulated Other Comprehensive Loss		Treasury Stock
Stockholders' Equity, Beginning Balance at Dec. 31, 2016		$ 232,852	$ 17,943	$ 270,880	$ 28,644	$ (400)	$ (6,834)		$ (77,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		21,255			21,255
Other Comprehensive Income (Loss)		699					699	[1]
Stock Dividend			538	16,661	(17,199)
Cash Dividends Paid	[2]	(10,119)			(10,119)
Stock Options Exercised, Net		734		335					399
Shares Issued Under the Directors' Stock Plan		126		84					42
Shares Issued Under the Employee Stock Purchase Plan		351		230					121
Shares Issued for Dividend Reinvestment Plans		1,256		843					413
Stock-Based Compensation Expense		261		261
Purchases of Treasury Stock		(2,767)							(2,767)
Stockholders' Equity, Ending Balance at Sep. 30, 2017		244,648	18,481	289,294	22,581	(400)	(6,135)		(79,173)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of change in accounting policy	[1]						(331)
Impact of change in accounting policy | Accounting Standards Update 2014-09		(102)			(102)
Impact of change in accounting policy | Accounting Standards Update 2016-01		0			331		(331)
Stockholders' Equity, Beginning Balance at Dec. 31, 2017		249,603	18,481	290,219	28,818	(200)	(8,514)		(79,201)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		27,521			27,521
Other Comprehensive Income (Loss)		(3,776)					(3,776)	[1]
Stock Dividend		0	554	21,126	(21,680)
Cash Dividends Paid		(10,630)			(10,630)
Stock Options Exercised, Net		1,977		942					1,035
Shares Issued Under the Directors' Stock Plan		103		72					31
Shares Issued Under the Employee Stock Purchase Plan		368		247					121
Shares Issued for Dividend Reinvestment Plans		1,302		890					412
Stock-Based Compensation Expense		267		267
Purchases of Treasury Stock		(1,823)							(1,823)
Stockholders' Equity, Ending Balance at Sep. 30, 2018		$ 264,810	$ 19,035	$ 313,763	$ 24,258	$ (200)	$ (12,621)		$ (79,425)

[1] All amounts are net of tax.

[2] Cash dividends paid per share have been adjusted for the September 27, 2018 3% stock dividend.