Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair value of assets and liabilities measured on recurring basis
Short-term investments	$ 26.7	$ 34.6
Forward contracts	11.3	2.3
Total	38.0	36.9
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of assets and liabilities measured on recurring basis
Short-term investments	26.7	34.6
Total	26.7	34.6
Significant Observable Inputs (Level 2)
Fair value of assets and liabilities measured on recurring basis
Forward contracts	11.3	2.3
Total	$ 11.3	$ 2.3