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Pension and Postretirement Health and Life Benefits (Schedule of Healthcare Cost Trend Rates) (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]    
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year 6.50% 7.00%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 5.00% 5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate 2021