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Retirement Benefits (Benefit Assumptions) (Details)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year 6.73% 7.16%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 4.96% 5.39%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate 2021 2018
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 3.65% 3.59%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.60% 3.65%
Expected long term return on assets, tax exempt accounts 3.50% 7.50%
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.34% 2.38%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.06% 2.79%
Expected long term return on assets, tax exempt accounts 7.00% 7.00%
Expected long term return on assets, taxable accounts 4.50% 4.50%