Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair value of assets and liabilities measured on recurring basis
Short-term investments	$ 37.4	$ 138.6
Forward contracts	2.5	8.4
Total	39.9	147.0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of assets and liabilities measured on recurring basis
Short-term investments	37.4	138.6
Total	37.4	138.6
Significant Observable Inputs (Level 2)
Fair value of assets and liabilities measured on recurring basis
Forward contracts	2.5	8.4
Total	$ 2.5	$ 8.4