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Stock-Based Compensation, Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]            
Options Outstanding, Number of Options (in shares) 286,000          
Options Outstanding, Weighted Average Remaining Term 2 years          
Options Outstanding, Weighted Average Exercise Price (in dollars per share) $ 50.14          
Options Exercisable, Number of Options (in shares) 286,000          
Options Exercisable, Weighted Average Exercise Price (in dollars per share) $ 50.14          
$39.53 to $40.02 [Member]            
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]            
Range of Exercise Prices, Lower Range Limit (in dollars per share) 39.53          
Range of Exercise Prices, Upper Range Limit (in dollars per share) $ 40.02          
Options Outstanding, Number of Options (in shares) 78,000          
Options Outstanding, Weighted Average Remaining Term 1 year 6 months          
Options Outstanding, Weighted Average Exercise Price (in dollars per share) $ 39.69          
Options Exercisable, Number of Options (in shares) 78,000          
Options Exercisable, Weighted Average Exercise Price (in dollars per share) $ 39.69          
$48.06 to $49.55 [Member]            
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]            
Range of Exercise Prices, Lower Range Limit (in dollars per share) 48.06          
Range of Exercise Prices, Upper Range Limit (in dollars per share) $ 49.55          
Options Outstanding, Number of Options (in shares) 88,000          
Options Outstanding, Weighted Average Remaining Term 1 year 2 months 12 days          
Options Outstanding, Weighted Average Exercise Price (in dollars per share) $ 48.75          
Options Exercisable, Number of Options (in shares) 88,000          
Options Exercisable, Weighted Average Exercise Price (in dollars per share) $ 48.75          
$55.99 to $59.70 [Member]            
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]            
Range of Exercise Prices, Lower Range Limit (in dollars per share) 55.99          
Range of Exercise Prices, Upper Range Limit (in dollars per share) $ 59.70          
Options Outstanding, Number of Options (in shares) 120,000          
Options Outstanding, Weighted Average Remaining Term 2 years 10 months 24 days          
Options Outstanding, Weighted Average Exercise Price (in dollars per share) $ 57.89          
Options Exercisable, Number of Options (in shares) 120,000          
Options Exercisable, Weighted Average Exercise Price (in dollars per share) $ 57.89          
Stock Options [Member]            
Estimated weighted average fair value for options granted and significant weighted average assumptions used [Abstract]            
Fair Value of Options on Grant Date (in dollars per share)         $ 14.77  
Options [Roll Forward]            
Outstanding at Beginning of Year (in shares) 372,000 611,000 1,429,000      
Granted (in shares) 0 0 0 0    
Exercised (in shares) (34,000) (229,000) (788,000)      
Expired or Forfeited (in shares) (52,000) (10,000) (30,000)      
Outstanding at End of Year (in shares) 286,000 372,000 611,000 1,429,000    
Exercisable at End of Year (in shares) 286,000 372,000 530,000      
Vested and Expected to Vest in the Future at End of Year (in shares) 286,000 372,000 599,000      
Weighted Average Exercise Price [Abstract]            
Outstanding at Beginning of Year (in dollars per share) $ 49.70 $ 48.88 $ 47.39      
Granted (in dollars per share) 0 0 0      
Exercised (in dollars per share) 38.32 47.21 45.97      
Expired or Forfeited (in dollars per share) 54.57 56.97 54.24      
Outstanding at End of Year (in dollars per share) 50.14 49.70 48.88 $ 47.39    
Exercisable at End of Year (in dollars per share) 50.14 49.70 47.43      
Vested and Expected to Vest in the Future at End of Year (in dollars per share) $ 50.14 $ 49.70 $ 48.90      
Weighted Average Remaining Term [Abstract]            
Outstanding at End of Year 2 years          
Exercisable at End of Year 2 years          
Vested and Expected to Vest in the Future at End of Year 2 years          
Average Intrinsic Value [Abstract]            
Outstanding at End of Year $ 0.0          
Exercisable at End of Year 0.0          
Vested and Expected to Vest in the Future at End of Year 0.0          
Options, Additional Disclosure [Abstract]            
Total intrinsic value of options exercised 0.3 $ 4.4 $ 10.4      
Total grant date fair value of stock options vested   $ 4.8        
Unrecognized share-based compensation expense $ 0.0          
Stock Options [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Exercise price of stock options granted as percentage of fair market value of stock at date of grant as required by the plan 100.00%          
Stock Options [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Exercisable period 10 years          
Stock Options [Member] | Vesting on Fourth Anniversary Date Following Date of Grant [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage           50.00%
Stock Options [Member] | Vesting on Fifth Anniversary Date Following Date of Grant [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage           50.00%
Stock Options [Member] | Annual Vesting on April 30th [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage         25.00%