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Share-Based Payments - Narrative (Details) - USD ($)
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares available for future issuance 5,174,159    
Forfeiture period for share-based payments 4 years    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period 35,002    
Closing market price (in dollars per share) $ 114.08    
Share-based compensation expense $ 15,400,000 $ 22,000,000 $ 34,600,000
Related income tax benefit $ (4,600,000) (7,200,000) (10,200,000)
Weighted-average period of recognition 3 years 4 months 24 days    
Equity instruments other than options vested in period weighted average grant date fair value total value $ 17,100,000 24,600,000 18,700,000
Fair value of equity awards other than stock options vesting $ 20,700,000 $ 32,700,000 $ 24,400,000
Restricted stock, vesting period (in Years) 1 year    
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted in period gross 0 0 370,000
Forfeiture period for share-based payments     10 years
Aggregate intrinsic value - outstanding $ 1,600,000    
Aggregate intrinsic value - exercisable $ 500,000    
Average remaining contractual term - outstanding 7 years    
Average remaining contractual term - exercisable 7 years    
Weighted average fair value of stock options granted     $ 18.67
Intrinsic value of options exercised $ 600,000 $ 0 $ 100,000
Share-based compensation expense 400,000 (1,000,000) 8,100,000
Related income tax benefit (100,000) 400,000 (3,000,000)
Unrecognized compensation expense 0    
Cash received from option exercises $ 3,900,000 $ 0 $ 100,000
Employee Stock Option [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period for share-based payments 1 year    
Employee Stock Option [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period for share-based payments 3 years    
Restricted Shares And Deferred Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock, vesting period (in Years) 4 years