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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, 2018 | Oct. 31, 2017 | Oct. 31, 2018 | Oct. 31, 2017 | Apr. 30, 2018
Investments Debt And Equity Securities [Abstract] |  |  |  |  |
Obligations for which assets are held in trust | $ 115,900 |  | $ 115,900 |  | $ 118,200
Unvested obligations under deferred compensation plans | 24,200 |  | 24,200 |  | $ 29,500
Gain (loss) on marketable securities | $ (4,800) | $ 3,400 | $ (836) | $ 6,797 |

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