Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 92.5	$ 81.7	$ 277.8	$ 243.4
Interest cost	84.5	106.6	253.6	319.0
Expected return on plan assets	(237.5)	(229.5)	(712.7)	(671.4)
Amortization of prior service cost	1.0	1.1	3.2	3.2
Recognized actuarial loss	121.9	58.4	366.3	286.5
Net periodic benefit cost	62.4	18.3	188.2	180.7
Retiree Health Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	12.4	10.2	37.0	30.6
Interest cost	8.1	10.9	24.4	32.8
Expected return on plan assets	(36.5)	(40.3)	(109.6)	(115.1)
Amortization of prior service cost	(14.9)	(21.9)	(44.7)	(51.6)
Recognized actuarial loss	0.8	0.7	2.4	2.1
Net periodic benefit cost	$ (30.1)	$ (40.4)	$ (90.5)	$ (101.2)