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Income Taxes (Details 5) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Reconciliation of unrecognized tax benefits      
Balance at May 1, $ 40.4 $ 46.3 $ 45.0
Increases:      
Current year tax positions 1.1 0.7 3.3
Prior year tax positions 0.5 1.2 0.2
Acquired businesses 0.0 0.0 3.3
Decreases:      
Prior year tax positions 0.0 0.9 0.9
Settlement with tax authorities 3.0 1.1 2.5
Expiration of statute of limitations periods 6.7 5.8 2.1
Balance at April 30, $ 32.3 $ 40.4 $ 46.3