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Pension and Other Postretirement Benefits (Schedule of Assumed Health Care Cost Trend Rates) (Details) - Other Postretirement Benefits
12 Months Ended
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]  
Health care cost trend rate assumed for 2018 4.60%
Rate that the cost trend rate gradually declines to 3.80%
Year that the rate reaches the rate it is assumed to remain 2035