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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

X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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-Publisher FASB
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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