Retirement Benefits - Schedule of Components of Retirement Benefit Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	$ 12.5	$ 14.4	$ 24.9	$ 28.8
Assumed return on plan assets	(15.1)	(12.4)	(30.1)	(24.8)
Amortization of prior service costs	0.1	0.1	0.1	0.1
Recognized net actuarial losses (gains)	17.6	16.9	35.3	33.9
Retirement benefits expense (income)	15.1	19.0	30.2	38.0
Pension Benefits | Restatement adjustment
Defined Benefit Plan Disclosure [Line Items]
Assumed return on plan assets		(3.8)		(7.5)
Reclassification of service cost to assumed return on plan assets		3.8		7.5
Postretirement Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	0.3	0.3	0.6	0.7
Assumed return on plan assets	0.0	0.0	0.0	0.0
Amortization of prior service costs	0.0	0.0	(0.1)	(0.1)
Recognized net actuarial losses (gains)	(1.0)	(1.0)	(1.9)	(2.0)
Retirement benefits expense (income)	$ (0.7)	$ (0.7)	$ (1.4)	$ (1.4)