Postretirement Benefit Plans (Amounts Recognized in Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Postretirement Health And Life Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit liability	$ 508	$ 540
Director's Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit liability	$ 1,116	$ 1,122