Deferred Compensation and Retirement Plans - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012	Apr. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Gross CSV of COLI contracts	$ 156,800,000		$ 156,800,000		$ 151,100,000
Outstanding policy loan	73,300,000		73,300,000		73,300,000
Increased in market value of COLI investments	2,000,000	2,200,000	4,236,000	3,256,000
Company's contributions			20,000,000
Executive Capital Accumulation Plan
Defined Benefit Plan Disclosure [Line Items]
Company's contributions	700,000	200,000	20,000,000	15,500,000
Deferred compensation arrangement vesting terms	Participants generally vest in Company contributions over a four year period.
Gain (loss) on investment	$ 2,900,000	$ 1,600,000	$ 3,400,000	$ (2,000,000)