Financial Instruments and Risk Management (Effect of Derivative Instruments on the Condensed Consolidated Balance Sheets Fair Values of Derivative Instruments Derivatives Not Designated As Hedging Instrument) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair value of financial instruments	$ 7.1	$ 88.9
Fair value of financial instruments	26.5	31.1
Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Fair value of financial instruments	7.1	9.3
Not Designated as Hedging Instruments | Prepaid expenses and other current assets | Foreign currency forward contracts
Derivatives, Fair Value [Line Items]
Fair value of financial instruments	7.1	9.3
Not Designated as Hedging Instruments | Other current liabilities | Foreign currency forward contracts
Derivatives, Fair Value [Line Items]
Fair value of financial instruments	$ 26.5	$ 31.1