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Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Defined Benefit Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 92.0 $ 78.7
Interest cost 84.3 105.0
Expected return on plan assets (238.0) (221.2)
Amortization of prior service cost 1.1 1.1
Recognized actuarial loss 121.8 111.0
Net periodic benefit cost 61.2 74.6
Retiree Health Benefit Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 11.7 9.8
Interest cost 8.1 11.0
Expected return on plan assets (36.6) (37.4)
Amortization of prior service cost (14.9) (14.9)
Recognized actuarial loss 0.8 0.8
Net periodic benefit cost $ (30.9) $ (30.7)