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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Oct. 31, 2019 | Oct. 31, 2018 | Oct. 31, 2019 | Oct. 31, 2018 | Apr. 30, 2019
Investments Debt And Equity Securities [Abstract] |  |  |  |  |
Obligations for which assets are held in trust | $ 131,500 |  | $ 131,500 |  | $ 122,300
Gain (Loss) on marketable securities | 1,200 | $ (4,800) | 3,121 | $ (836) |
Unvested obligations under deferred compensation plans | $ 23,200 |  | $ 23,200 |  | $ 24,600

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