Retirement Benefits - Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net asset value per share practical expedient(a)	$ 20.3	$ 17.4
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	202.9	173.4	$ 164.8
Non-U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 182.6	$ 156.0