Quarterly report pursuant to Section 13 or 15(d)

Deferred Compensation and Retirement Plans - Additional Information (Detail)

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Deferred Compensation and Retirement Plans - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Oct. 31, 2015
Oct. 31, 2014
Oct. 31, 2015
Oct. 31, 2014
Apr. 30, 2015
Defined Benefit Plan Disclosure [Line Items]          
Increase in market value of the underlying COLI investments     $ 3,295,000 $ 5,494,000  
Gain (loss) on marketable securities $ (2,500,000) $ 2,500,000 (1,818,000) 4,527,000  
CSV of Coli Contracts          
Defined Benefit Plan Disclosure [Line Items]          
Gross CSV 173,900,000   173,900,000   $ 172,300,000
Outstanding policy loans 68,500,000   68,500,000   $ 69,600,000
Increase in market value of the underlying COLI investments 800,000 2,200,000 3,295,000 5,494,000  
Executive Capital Accumulation Plan          
Defined Benefit Plan Disclosure [Line Items]          
Company's contributions 20,000,000 17,400,000 $ 22,000,000 $ 18,600,000  
Deferred compensation arrangement vesting period     4 years 4 years  
(Loss) gain on investment in the deferred compensation plan $ (1,600,000) $ 1,000,000 $ (900,000) $ 2,700,000