Retirement Benefits - Schedule of Components of Retirement Benefit Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.7	$ 3.5
Interest cost on benefit obligation	14.4	16.0
Assumed return on plan assets	(16.1)	(17.5)
Amortization of prior service credits	0.0	0.0
Recognized net actuarial losses (gains)	17.0	15.9
Retirement benefit expense (benefit)	19.0	17.9
Postretirement Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on benefit obligation	0.4	0.5
Assumed return on plan assets	0.0	0.0
Amortization of prior service credits	(0.1)	(0.3)
Recognized net actuarial losses (gains)	(1.0)	(0.9)
Retirement benefit expense (benefit)	$ (0.7)	$ (0.7)