Deferred Compensation and Retirement Plans - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended
Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011	Apr. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Gross CSV of COLI contracts	$ 153,800,000		$ 153,800,000		$ 151,100,000
Outstanding policy loan	73,300,000		73,300,000		73,300,000
Increased in market value of COLI investments	1,500,000	200,000	2,178,000	1,047,000
Company's contributions			19,300,000
Executive Capital Accumulation Plan
Defined Benefit Plan Disclosure [Line Items]
Company's contributions	1,800,000	3,000,000	19,300,000	15,300,000
Deferred compensation arrangement vesting terms	Participants generally vest in Company contributions over a four year period.
Gain (loss) on investment	$ 1,500,000	$ (2,300,000)	$ 500,000	$ (3,600,000)

73,300,000

19,300,000

Participants generally vest in Company contributions over a four year period.