Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 29,326	$ 26,534	$ 24,662
Other Comprehensive Income (Loss), Net of Tax:
Unrealized Net Securities Holding Losses Arising During the Year	(940)	(1,024)	(1,832)
Reclassification Adjustment for Net Securities Losses (Gains) Included in Net Income	337	13	(78)
Net Retirement Plan Gain	214	1,721	848
Net Retirement Plan Prior Service (Cost) Credit	0	0	(224)
Amortization of Net Retirement Plan Actuarial Loss	362	435	514
Accretion of Net Retirement Plan Prior Service Credit	(8)	(7)	(34)
Other Comprehensive Income (Loss)	(35)	1,138	(806)
Comprehensive Income	$ 29,291	$ 27,672	$ 23,856