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Retirement Benefits Retirement Benefits - Pension and Other Postretirement Benefit Plan Target Asset Allocations (Details)
Dec. 31, 2017
Dec. 31, 2016
Pension Plan, Defined Benefit [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 100.00% 100.00%
Pension Plan, Defined Benefit [Member] | Equity Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation percentage 35.00%  
Percentage of Plan Assets 35.00% 33.00%
Pension Plan, Defined Benefit [Member] | Fixed Income Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation percentage 38.00%  
Percentage of Plan Assets 39.00% 39.00%
Pension Plan, Defined Benefit [Member] | Alternative Investments [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation percentage [1] 27.00%  
Percentage of Plan Assets [1] 26.00% 28.00%
Other Postretirement Benefit Plan, Defined Benefit [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 100.00% 100.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Equity Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation percentage 46.00%  
Percentage of Plan Assets 47.00% 47.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Fixed Income Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation percentage 28.00%  
Percentage of Plan Assets 28.00% 29.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Alternative Investments [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation percentage [1] 26.00%  
Percentage of Plan Assets [1] 25.00% 24.00%
[1] Alternative investments include private equity, hedge funds, real estate, and private credit.