Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Unfunded pension obligation	$ 540.8		$ 548.2
Cash contributions expected to make	72.0
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	35.0
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 funding to tax-qualified defined benefit pension plan, recoverable in current year	$ 9.3	$ 6.9