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Fair Value Measurements - Schedule of Financial Assets and Liabilities (Details) - Recurring - USD ($) $ in Thousands | Oct. 31, 2018 | Jan. 31, 2018
--- | --- | ---
Short-term investments: |  |
Total financial assets | $ 383,574 | $ 262,990
Level 1 |  |
Short-term investments: |  |
Total financial assets | 383,574 | 262,990
Level 2 |  |
Short-term investments: |  |
Total financial assets | 0 | 0
Level 3 |  |
Short-term investments: |  |
Total financial assets | 0 | 0
U.S. government treasury securities |  |
Short-term investments: |  |
Short-term investments: | 382,681 | 217,072
U.S. government treasury securities | Level 1 |  |
Short-term investments: |  |
Short-term investments: | 382,681 | 217,072
U.S. government treasury securities | Level 2 |  |
Short-term investments: |  |
Short-term investments: | 0 | 0
U.S. government treasury securities | Level 3 |  |
Short-term investments: |  |
Short-term investments: | 0 | 0
Money Market Funds |  |
Cash and cash equivalents: |  |
Cash and cash equivalents: | 893 | 45,918
Money Market Funds | Level 1 |  |
Cash and cash equivalents: |  |
Cash and cash equivalents: | 893 | 45,918
Money Market Funds | Level 2 |  |
Cash and cash equivalents: |  |
Cash and cash equivalents: | 0 | 0
Money Market Funds | Level 3 |  |
Cash and cash equivalents: |  |
Cash and cash equivalents: | $ 0 | $ 0

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