Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Jan. 31, 2020	Jan. 31, 2019
Short-term investments:
Total financial assets	$ 904,182	$ 406,154
Level 1
Short-term investments:
Total financial assets	904,182	406,154
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:
U.S. government treasury securities	280,326	318,139
U.S. government treasury securities | Level 1
Short-term investments:
U.S. government treasury securities	280,326	318,139
U.S. government treasury securities | Level 2
Short-term investments:
U.S. government treasury securities	0	0
U.S. government treasury securities | Level 3
Short-term investments:
U.S. government treasury securities	0	0
Money market funds
Cash and cash equivalents:
Money market funds	623,856	88,015
Money market funds | Level 1
Cash and cash equivalents:
Money market funds	623,856	88,015
Money market funds | Level 2
Cash and cash equivalents:
Money market funds	0	0
Money market funds | Level 3
Cash and cash equivalents:
Money market funds	$ 0	$ 0