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Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jul. 31, 2019 | Jul. 31, 2018 | Jul. 31, 2019 | Jul. 31, 2018
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Sales and maturities of marketable securities |  |  | $ 320,411 | $ 130,029
Amortization of discounts and premiums, net | $ 152 | $ 517 | 271 | 1,151
Other Income, Net |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Net realized gain (loss) | $ 19 | $ 0 | $ 26 | $ (13)

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