Retirement Benefits - Schedule of Components of Retirement Benefit Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.7	$ 3.6	$ 11.2	$ 10.5
Interest cost on benefit obligation	14.4	16.0	43.2	48.1
Assumed return on plan assets	(16.1)	(17.5)	(48.4)	(52.6)
Amortization of prior service credits	0.0	0.0	0.1	0.1
Recognized net actuarial losses (gains)	17.0	15.9	50.9	47.8
Retirement benefit expense (benefit)	19.0	18.0	57.0	53.9
Postretirement Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost on benefit obligation	0.4	0.5	1.1	1.4
Assumed return on plan assets	0.0	0.0	0.0	0.0
Amortization of prior service credits	0.0	(0.3)	(0.1)	(0.9)
Recognized net actuarial losses (gains)	(1.1)	(0.9)	(3.1)	(2.7)
Retirement benefit expense (benefit)	$ (0.7)	$ (0.7)	$ (2.1)	$ (2.2)