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Retirement Plans and Postretirement Medical Benefits (Effect of Change in the Assumed Health Care Cost Trend Rates) (Details) - Nonpension postretirement benefit plans
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect of 1% increase on total of service and interest cost components $ 330
Effect of 1% decrease on total of service and interest cost components (276)
Effect of 1% increase on postretirement benefit obligation 7,295
Effect of 1% decrease on postretirement benefit obligation $ (6,212)