Short-Term Investments - Summary of Estimated Fair Value of Short-Term Investments, Designated as Available-for-Sale and Classified by Contractual Maturity (Detail) - USD ($) $ in Thousands |
Apr. 30, 2018 |
Jan. 31, 2018 |
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|---|---|---|---|---|---|
| Investments Debt And Equity Securities [Abstract] | |||||
| Due in one year or less | $ 282,866 | $ 308,172 | |||
| Due in greater than one year | 175,203 | 133,607 | |||
| Total | $ 458,069 | $ 441,779 | [1] | ||
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