Annual report pursuant to Section 13 and 15(d)

Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail)

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Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Investment Holdings [Line Items]      
Obligations for which assets are held in trust $ 137,100 $ 138,800  
Gain (loss) on marketable securities $ 10,842 $ (3,333) $ 8,829