Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments
Maximum Remaining Maturity of Foreign Currency Derivatives	6 months
Notes Payable 4.5 Percent Due 2021 [Member]
Derivative Instruments
Debt instrument, face amount	$ 400.0
Fixed-rate (as a percent)	4.50%
Derivatives not designated as hedging instruments | Forward exchange contracts
Derivative Instruments
Notional amounts of derivatives	$ 2.7	$ 4.1
Derivatives designated as hedging instruments | Interest-rate swap
Derivative Instruments
Notional amounts of derivatives	$ 400.0
Number of swap agreements	4
(Level 2) | Measured on a recurring basis | Interest-rate swap
Derivative Instruments
Derivative Liability	$ 0.6	0.0
Derivative Asset	$ 0.0	$ 1.3