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Retirement Benefits (Components of Net Periodic (Benefit) Cost) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Defined Benefit Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 83.8 $ 70.4
Interest cost 165.0 161.1
Expected return on plan assets (277.6) (263.3)
Amortization of prior service cost 0.5 0.6
Recognized actuarial gain (loss) 30.6 30.0
Net periodic (benefit) cost 2.3 (1.2)
Retiree Health Benefit Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 8.2 7.7
Interest cost 15.5 15.4
Expected return on plan assets (48.1) (45.5)
Amortization of prior service cost (1.4) (13.2)
Recognized actuarial gain (loss) (0.6) (0.9)
Net periodic (benefit) cost $ (26.4) $ (36.5)