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Marketable Securities - Additional Information (Detail) (USD $)
 3 Months Ended 6 Months Ended
Oct. 31, 2014
Oct. 31, 2013
Oct. 31, 2014
Oct. 31, 2013
Apr. 30, 2014
Schedule Of Marketable Securities [Line Items]          
Proceeds from sales/maturities of available-for-sale securities $ 0 $ 4,000,000 $ 2,000,000 $ 33,300,000  
Trading securities 135,603,000 [1],[2]   135,603,000 [1],[2]   116,207,000 [1],[2]
Gross unrealized gains 8,800,000   8,800,000   9,200,000
Gross unrealized losses $ 500,000   $ 500,000   $ 700,000
Minimum
          
Schedule Of Marketable Securities [Line Items]          
Available-for-sale marketable securities, remaining maturities     2 months    
Maximum
          
Schedule Of Marketable Securities [Line Items]          
Available-for-sale marketable securities, remaining maturities     1 year 2 months 12 days    
[1] 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 October 31, 2014 and April 30, 2014, the Company had no investments classified as Level 3.
[2] (1) These investments are held in trust for settlement of the Company's vested and unvested obligations of $135.9 million and $117.6 million as of October 31, 2014 and April 30, 2014, respectively, under the ECAP (see Note 6 - Deferred Compensation and Retirement Plans). During the three and six months ended October 31, 2014, the fair value of the investments increased; therefore, the Company recognized income of $2.5 million and $4.5 million, respectively, which was recorded in other income, net. During the three and six months ended October 31, 2013, the fair value of the investments increased; therefore, the Company recognized income of $4.4 million and $6.4 million, respectively, which was recorded in other income, net.