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Retirement Benefits - Components of Net Periodic (Benefit) Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Defined Benefit Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 84.9 $ 72.5 $ 254.3 $ 217.8
Interest cost 165.9 162.3 496.6 486.0
Expected return on plan assets (278.8) (264.0) (834.3) (791.5)
Amortization of prior service cost 0.6 0.6 1.6 1.8
Recognized actuarial gain (loss) 31.4 30.6 93.8 91.5
Net periodic cost 4.0 2.0 12.0 5.6
Retiree Health Benefit Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 8.9 7.9 26.6 23.8
Interest cost 15.5 15.3 46.6 46.0
Expected return on plan assets (48.0) (45.5) (144.1) (136.6)
Amortization of prior service cost (1.4) (13.3) (4.2) (39.7)
Recognized actuarial gain (loss) (0.6) (1.5) (1.9) (4.4)
Net periodic cost $ (25.6) $ (37.1) $ (77.0) $ (110.9)