Retirement Benefits - Components of Net Periodic (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 85.6	$ 74.9	$ 169.4	$ 145.3
Interest cost	165.7	162.6	330.7	323.7
Expected return on plan assets	(277.9)	(264.2)	(555.5)	(527.5)
Amortization of prior service cost	0.5	0.6	1.0	1.2
Recognized actuarial gain (loss)	31.8	30.9	62.4	60.9
Net periodic cost	5.7	4.8	8.0	3.6
Retiree Health Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	9.5	8.2	17.7	15.9
Interest cost	15.6	15.3	31.1	30.7
Expected return on plan assets	(48.0)	(45.6)	(96.1)	(91.1)
Amortization of prior service cost	(1.4)	(13.2)	(2.8)	(26.4)
Recognized actuarial gain (loss)	(0.7)	(2.0)	(1.3)	(2.9)
Net periodic cost	$ (25.0)	$ (37.3)	$ (51.4)	$ (73.8)