Marketable Securities - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
	Jan. 31, 2013		Apr. 30, 2012
Schedule Of Marketable Securities [Line Items]
Trading securities	$ 95,807,000	[1],[2]	$ 82,176,000	[1],[2]
Minimum
Schedule Of Marketable Securities [Line Items]
Available for sale marketable securities, remaining maturities	1 month
Maximum
Schedule Of Marketable Securities [Line Items]
Available for sale marketable securities, remaining maturities	3 years
Debt Securities
Schedule Of Marketable Securities [Line Items]
Gross unrealized gains	5,100,000		3,500,000
Gross unrealized losses			$ 400,000

[1] (1) These investments are held in trust for settlement of the Company's obligations of $96.5 million and $82.6 million as of January 31, 2013 and April 30, 2012, respectively, under the Executive Capital Accumulation Plan ("ECAP") (see Note 6 - Deferred Compensation and Retirement Plans). The fair value of marketable securities classified as trading (held in trust to satisfy obligations under the ECAP) increased by $4.6 million and decreased by $2.3 million during the nine months ended January 31, 2013 and 2012, respectively, recorded in other income (loss), net on the consolidated statements of income. The remaining activity is comprised primarily of Company and employee contributions made of $20.0 million and $1.9 million, respectively, offset by distributions of $12.9 million for the nine months ended January 31, 2013.

[2] The Company's financial assets measured at fair value on a recurring basis include, trading securities classified as Level 1 and available-for-sale securities classified as Level 2. As of January 31, 2013 and April 30, 2012, the Company had cash equivalents of $58.5 million and $60.5 million and restricted cash of $2.9 million and $10.0 million, respectively, classified as Level 1.