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Retirement Benefits (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Curtailment $ 159.0    
Special termination benefit 354.7    
Total accumulated benefit obligation for our defined benefit pension plans 13,900.0 $ 11,490.0  
Expense under the plans $ 169.1 $ 175.0 $ 162.4
Percentage of global investments in plan assets (in percentage) 80.00%    
Expected contribution $ 50.0    
Marketable equity securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets, target allocation (in percentage) 80.00%    
Developed markets      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets, target allocation (in percentage) 20.00%