Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 78.7	$ 61.8
Interest cost	105.0	120.7
Expected return on plan assets	(221.2)	(211.1)
Amortization of prior service cost	1.1	1.5
Recognized actuarial loss	111.0	69.6
Net periodic benefit cost	74.6	42.5
Retiree Health Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	9.8	8.8
Interest cost	11.0	14.6
Expected return on plan assets	(37.4)	(36.0)
Amortization of prior service cost	(14.9)	(15.7)
Recognized actuarial loss	0.8	0.5
Net periodic benefit cost	$ (30.7)	$ (27.8)