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Derivative Instruments - Freestanding Derivative Forward Contracts (Details)
$ in Millions
12 Months Ended
Mar. 31, 2019
USD ($)
contract
Mar. 31, 2018
USD ($)
contract
Mar. 31, 2017
USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]      
Percentage of company sales denominated in Us dollars 99.00%    
Foreign currency transaction gains (loss), before tax $ 1.7 $ 9.3 $ 1.0
Number of foreign currency derivatives held | contract 0 0  
Foreign Exchange Forward      
Derivative [Line Items]      
Gain (loss) on foreign currency derivatives $ 0.0 $ 4.2 $ (2.3)