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Cash, Trading Securities and Foreign Currency Forward Contracts' Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value Recorded as Cash and Cash Equivalents, Current Portion of Current Marketable Securities, Non-Current Marketable Securities, or Other Accrued Liabilities (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Oct. 31, 2016
Oct. 31, 2015
Oct. 31, 2016
Oct. 31, 2015
Apr. 30, 2016
Investment Holdings [Line Items]          
Obligations for which assets are held in trust $ 133,700   $ 133,700   $ 138,800
Gain (loss) on marketable securities $ (700) $ (2,500) $ 3,226 $ (1,818)