Fair Value Measurements - Schedule of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 30, 2020	Jan. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain on securities	$ 1,000
Short-term investments:
Total financial assets	907,838	$ 904,182
Level 1
Short-term investments:
Total financial assets	907,838	904,182
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:	276,372	280,326
U.S. government treasury securities | Level 1
Short-term investments:
Short-term investments:	276,372	280,326
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:	631,466	623,856
Money market funds | Level 1
Cash and cash equivalents:
Cash and cash equivalents:	631,466	623,856
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