Cash Equivalents and Short-term Investments - Schedule of Amortized Costs, Unrealized Gains and Losses and Estimated Fair Value of Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Oct. 31, 2020	Jan. 31, 2020
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	$ 2,418,424	$ 1,318,359
Unrealized Gain	2,700	1,205
Unrealized Loss	(159)	(8)
Estimated Fair Value	2,420,965	1,319,556
U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	1,795,309	575,920
Unrealized Gain	1,978	686
Unrealized Loss	(130)	(8)
Estimated Fair Value	1,797,157	576,598
Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	287,523	305,859
Unrealized Gain	722	519
Unrealized Loss	(29)	0
Estimated Fair Value	288,216	306,378
Cash Equivalents
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	335,592	436,580
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	335,592	436,580
Cash Equivalents | Money market funds
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	335,592	416,584
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	335,592	416,584
Cash Equivalents | U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost		19,996
Unrealized Gain		0
Unrealized Loss		0
Estimated Fair Value		19,996
Short-term Investments
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	2,082,832	881,779
Unrealized Gain	2,700	1,205
Unrealized Loss	(159)	(8)
Estimated Fair Value	$ 2,085,373	$ 882,976