Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair value of assets and liabilities measured on recurring basis
Short-term investments	$ 24.1	$ 34.6
Forward contracts		2.3
Forward contracts	(8.1)
Total	16.0	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	24.1	34.6
Total	24.1	34.6
Significant Observable Inputs (Level 2)
Fair value of assets and liabilities measured on recurring basis
Forward contracts		2.3
Forward contracts	(8.1)
Total	$ (8.1)	$ 2.3