Quarterly report pursuant to Section 13 or 15(d)

Summary of Marketable Securities (Parenthetical) (Detail)

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Summary of Marketable Securities (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended
Jul. 31, 2015
Jul. 31, 2014
Apr. 30, 2015
Schedule Of Marketable Securities [Line Items]      
Obligations for which assets are held in trust $ 125,500   $ 129,100
Gain on marketable securities $ 665 $ 2,018