Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 83.0	$ 63.5	$ 161.7	$ 125.4
Interest cost	107.3	122.6	212.3	243.3
Expected return on plan assets	(220.6)	(210.1)	(441.8)	(421.2)
Amortization of prior service cost	1.1	1.5	2.2	3.0
Recognized actuarial loss	117.1	73.1	228.1	142.7
Net periodic benefit cost	87.9	50.6	162.5	93.2
Retiree Health Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	10.6	9.4	20.4	18.2
Interest cost	10.9	14.4	21.9	29.0
Expected return on plan assets	(37.4)	(36.0)	(74.8)	(71.9)
Amortization of prior service cost	(14.9)	(15.7)	(29.8)	(31.5)
Recognized actuarial loss	0.6	0.4	1.4	0.9
Net periodic benefit cost	$ (30.2)	$ (27.5)	$ (60.9)	$ (55.3)