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Preferred and Common Stock (Details) - USD ($)
3 Months Ended 12 Months Ended 15 Months Ended
Apr. 30, 2019
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
May 31, 2018
Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award            
Shares issued of preferred stock (shares) 0 0 0      
Options outstanding (shares) 109,827 109,827        
Aggregate intrinsic value of outstanding options $ 9,660,000 $ 9,660,000        
Aggregate intrinsic value of exercised options   6,868,000        
Repurchase of common stock   $ 35,247,000 $ 215,434,000 $ 49,374,000    
Stock Options            
Share-based Compensation Arrangement by Share-based Payment Award            
Reduction in available shares per stock option issued (shares) 1 1        
Options outstanding (shares) 109,827 109,827 181,673 222,050   291,200
Weighted average remaining contractual life (years)   2 years 2 months 3 days        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 44.39 $ 44.39 $ 39.48 $ 38.51   $ 37.46
Restricted Stock            
Share-based Compensation Arrangement by Share-based Payment Award            
Reduction in available shares per restricted stock or restricted stock unit issued (shares) 2 2        
Restricted Stock Units            
Share-based Compensation Arrangement by Share-based Payment Award            
Reduction in available shares per restricted stock or restricted stock unit issued (shares) 2 2        
Stock Options, Restricted Stock and Restricted Stock Units            
Share-based Compensation Arrangement by Share-based Payment Award            
Share-based compensation expense   $ 16,410,000 $ 18,800,000 $ 10,697,000    
2009 Stock Incentive Plan            
Share-based Compensation Arrangement by Share-based Payment Award            
Shares available for grant under the Plan (shares) 2,984,032 2,984,032        
Unrecognized compensation costs $ 7,682,000 $ 7,682,000        
2017 Stock Repurchase Plan            
Share-based Compensation Arrangement by Share-based Payment Award            
Share repurchase program authorized amount     300,000,000   $ 300,000,000  
Share repurchase program period in force   2 years        
Common stock shares repurchased (shares)         2,794,192  
2018 Stock Repurchase Plan            
Share-based Compensation Arrangement by Share-based Payment Award            
Share repurchase program authorized amount     $ 300,000,000      
Repurchase of common stock $ 0          
Preferred Stock            
Share-based Compensation Arrangement by Share-based Payment Award            
Authorized shares of preferred stock (shares) 1,000,000 1,000,000        
Shares issued of preferred stock (shares) 0 0        
Series A Preferred Stock            
Share-based Compensation Arrangement by Share-based Payment Award            
Authorized shares of preferred stock (shares) 250,000 250,000        
Common Stock            
Share-based Compensation Arrangement by Share-based Payment Award            
Authorized shares of common stock (shares) 120,000,000 120,000,000