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Derivative Instruments and Hedging Activities (Details)
£ in Millions, $ in Millions
3 Months Ended 6 Months Ended
Oct. 31, 2017
USD ($)
Oct. 31, 2016
USD ($)
Contract
Oct. 31, 2017
USD ($)
Oct. 31, 2016
USD ($)
Contract
Apr. 30, 2017
USD ($)
Oct. 31, 2016
GBP (£)
Contract
Derivative [Line Items]            
Variable rate loans outstanding $ 563.0 $ 884.0 $ 563.0 $ 884.0    
Interest Rate Swaps [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Expense [Member]            
Derivative [Line Items]            
Net gain (losses) reclassified from Accumulated Other Comprehensive Loss 0.3 0.4 (0.4) (0.9)    
Interest Rate Swaps [Member] | Recurring [Member] | Level 2 [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]            
Derivative [Line Items]            
Assets fair value of derivative instrument $ 3.9   $ 3.9   $ 3.9  
Liability fair value of derivative instrument   $ 0.2   $ 0.2    
Interest Rate Swaps [Member] | April 2016 Interest Rate Swap (Variable Rate Loans) [Member] | LIBOR [Member]            
Derivative [Line Items]            
Inception date     Apr. 04, 2016      
Fixed interest rate to be paid 0.92%   0.92%      
Description of variable rate basis     one-month LIBOR      
Term of variable rate     1 month      
Term of derivative instrument     3 years      
Expiration date     May 15, 2019      
Notional amount of derivative liability $ 350.0   $ 350.0      
Forward Exchange Contracts [Member] | Not Designated as Hedging Instrument [Member]            
Derivative [Line Items]            
Notional amount of derivative liability | £           £ 274
Number of open derivative contracts held | Contract   1   1   1
Forward Exchange Contracts [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Transaction Gains (Losses) [Member]            
Derivative [Line Items]            
Gain (loss) on fair value of derivative instruments   $ 25.4   $ 64.7    
Forward Exchange Contracts [Member] | Recurring [Member] | Level 2 [Member] | Not Designated as Hedging Instrument [Member] | Prepaid and Other Current Assets [Member]            
Derivative [Line Items]            
Gain recognized on derivative instruments   $ 66.0   $ 66.0