Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer		$ 12.6
Expected contributions in current fiscal year	$ 0.8	$ 0.8
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year			$ 5.5
Pension Benefits | U.S.
Defined Benefit Plan Disclosure [Line Items]
Employer discretionary contribution	$ 10.3
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year			$ 0.1