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Fair Value Measurements - Assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands | Jan. 31, 2021 | Jan. 31, 2020
--- | --- | ---
Available-for-sale securities |  |
Amortized Cost | $ 584,187 | $ 818,970
Gross Unrealized Gains | 388 | 1,204
Gross Unrealized Losses | (96) | (82)
Estimated Fair Value | 584,479 | 820,092
Cash | 281,700 | 75,800
Cash equivalents | Money market funds | Fair Value, Inputs, Level 1 |  |
Available-for-sale securities |  |
Amortized Cost | 284,312 | 165,424
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 284,312 | 165,424
Available-for-sale securities |  |
Available-for-sale securities |  |
Estimated Fair Value | 300,167 |
Available-for-sale securities | Fair Value, Inputs, Level 2 |  |
Available-for-sale securities |  |
Amortized Cost | 299,375 | 653,546
Gross Unrealized Gains | 388 | 1,204
Gross Unrealized Losses | (96) | (82)
Estimated Fair Value | 299,667 | 654,668
Available-for-sale securities | Commercial paper | Fair Value, Inputs, Level 2 |  |
Available-for-sale securities |  |
Amortized Cost | 42,048 | 14,919
Gross Unrealized Gains | 1 | 7
Gross Unrealized Losses | (23) | (1)
Estimated Fair Value | 42,026 | 14,925
Available-for-sale securities | Corporate notes and bonds | Fair Value, Inputs, Level 2 |  |
Available-for-sale securities |  |
Amortized Cost | 199,277 | 372,844
Gross Unrealized Gains | 375 | 891
Gross Unrealized Losses | (67) | (31)
Estimated Fair Value | 199,585 | 373,704
Available-for-sale securities | Corporate notes and bonds | Fair Value, Inputs, Level 3 |  |
Available-for-sale securities |  |
Amortized Cost | 500 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Estimated Fair Value | 500 |
Available-for-sale securities | U.S. Treasury securities | Fair Value, Inputs, Level 2 |  |
Available-for-sale securities |  |
Amortized Cost | 4,998 | 90,697
Gross Unrealized Gains | 0 | 153
Gross Unrealized Losses | 0 | (1)
Estimated Fair Value | 4,998 | 90,849
Available-for-sale securities | U.S. government agency securities | Fair Value, Inputs, Level 2 |  |
Available-for-sale securities |  |
Amortized Cost | 53,052 | 175,086
Gross Unrealized Gains | 12 | 153
Gross Unrealized Losses | (6) | (49)
Estimated Fair Value | $ 53,058 | $ 175,190

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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