Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Oct. 31, 2020	Jan. 31, 2020
Assets:
Cash equivalents, fair value	$ 335,592	$ 436,580
Short term investments, fair value	2,085,373	882,976
Total cash equivalents and short-term investments	2,420,965	1,319,556
U.S. treasury securities
Assets:
Cash equivalents, fair value		19,996
Short term investments, fair value	1,797,157	576,598
Corporate debt securities
Assets:
Short term investments, fair value	288,216	306,378
Money market funds
Assets:
Cash equivalents, fair value	335,592	416,584
Level 1
Assets:
Cash equivalents, fair value	335,592	416,584
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	335,592	416,584
Level 1 | U.S. treasury securities
Assets:
Cash equivalents, fair value		0
Short term investments, fair value	0	0
Level 1 | Corporate debt securities
Assets:
Short term investments, fair value	0	0
Level 1 | Money market funds
Assets:
Cash equivalents, fair value	335,592	416,584
Level 2
Assets:
Cash equivalents, fair value	0	19,996
Short term investments, fair value	2,085,373	882,976
Total cash equivalents and short-term investments	2,085,373	902,972
Level 2  | U.S. treasury securities
Assets:
Cash equivalents, fair value		19,996
Short term investments, fair value	1,797,157	576,598
Level 2  | Corporate debt securities
Assets:
Short term investments, fair value	288,216	306,378
Level 2  | Money market funds
Assets:
Cash equivalents, fair value	0	0
Level 3
Assets:
Cash equivalents, fair value	0	0
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	0	0
Level 3 | U.S. treasury securities
Assets:
Cash equivalents, fair value		0
Short term investments, fair value	0	0
Level 3 | Corporate debt securities
Assets:
Short term investments, fair value	0	0
Level 3 | Money market funds
Assets:
Cash equivalents, fair value	$ 0	$ 0