Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Unfunded pension obligation			$ 533.1		$ 533.1		$ 548.2
Cash funding to tax-qualified defined benefit pension plan, recoverable in current year					37.0
Cash funding to tax-qualified defined benefit pension plan, recoverable in future years					39.0
Employer contribution to defined benefit plan					18.6
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan assets, total							925.1
Projected benefit obligations, total							1,492.1
Unfunded pension obligation							$ 548.2
Cash contributions expected to make			76.0		76.0
Cash funding to tax-qualified defined benefit pension plan, recoverable in current year			$ 9.3	$ 6.9	$ 18.6	$ 13.8
Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Employer contribution to defined benefit plan	$ 8.8	$ 48.6