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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Jul. 31, 2018 | Jul. 31, 2017 | Apr. 30, 2018
Investments Debt And Equity Securities [Abstract] |  |  |
Obligations for which assets are held in trust | $ 125,400 |  | $ 118,200
Gain (loss) on marketable securities | $ 4,001 | $ 3,429 |

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