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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Apr. 30, 2019 | Apr. 30, 2018 | Apr. 30, 2017
Investments Debt And Equity Securities [Abstract] |  |  |
Obligations for which assets are held in trust | $ 122,300 | $ 118,200 |
Gain on marketable securities | 8,134 | 10,278 | $ 10,842
Unvested obligations under deferred compensation plans | $ 24,600 | $ 29,500 |

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