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Pensions and Other Postretirement Benefits (Accrued Pension and Other Postretirement Costs) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Accrued pension and other postretirement costs $ (16,424) $ (14,713)
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Accrued pension and other postretirement costs (11,163) (10,634)
OPEB Plans    
Defined Benefit Plan Disclosure [Line Items]    
Accrued pension and other postretirement costs $ (4,726) $ (3,833)