Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 61.8	$ 77.6
Interest cost	120.7	112.0
Expected return on plan assets	(211.1)	(211.0)
Amortization of prior service cost	1.5	1.2
Recognized actuarial loss	69.6	89.9
Net periodic benefit cost	42.5	69.7
Retiree Health Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	8.8	10.7
Interest cost	14.6	13.9
Expected return on plan assets	(36.0)	(43.9)
Amortization of prior service cost	(15.7)	(20.5)
Recognized actuarial loss	0.5	2.4
Net periodic benefit cost	$ (27.8)	$ (37.4)