Retirement Benefit Plans - Funded status of plans with ABO in excess of plan assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ (681.2)	$ (594.0)
Plan assets	494.8	423.3
Funded status of plan	(186.4)	(170.7)
Germany
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan	(75.7)	(77.5)
United States
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan	(43.2)	(53.0)
Other
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan	$ (67.5)	$ (40.2)