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Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Apr. 30, 2020 | Apr. 30, 2019
Schedule of Available-for-sale Securities [Line Items] |  |
Sales and maturities of marketable securities | $ 311,258 | $ 151,761
Amortization of discounts and premiums, net | 409 | 119
Other Income, Net |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Net realized gain (loss) | $ 454 | $ 7

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