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Financial Instruments - Financial Instruments and Balance Sheet Classification (Parenthetical) (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jan. 31, 2018 | Jan. 31, 2017 | Jan. 31, 2018 | Jan. 31, 2017 | Apr. 30, 2017
Investment Holdings [Line Items] |  |  |  |  |
Obligations for which assets are held in trust | $ 122,300 |  | $ 122,300 |  | $ 99,500
Gain (loss) on marketable securities | $ 7,200 | $ 3,900 | $ 14,022 | $ 7,090 |

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