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	Apr. 30, 2020	Jan. 31, 2020
Cash Equivalents and Short-term Investments [Abstract]
Amortized cost	$ 1,324,939	$ 1,318,359
Unrealized Gain	5,969	1,205
Unrealized Loss	(138)	(8)
Estimated Fair Value	1,330,770	1,319,556
Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized cost	246,587	305,859
Unrealized Gain	1,029	519
Unrealized Loss	(137)	0
Estimated Fair Value	247,479	306,378
U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized cost	575,143	575,920
Unrealized Gain	4,935	686
Unrealized Loss	(1)	(8)
Estimated Fair Value	580,077	576,598
Cash Equivalents
Cash Equivalents and Short-term Investments [Abstract]
Amortized cost	503,209	436,580
Unrealized Gain	5	0
Unrealized Loss	0	0
Estimated Fair Value	503,214	436,580
Cash Equivalents | Money market funds
Cash Equivalents and Short-term Investments [Abstract]
Amortized cost	497,208	416,584
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	497,208	416,584
Cash Equivalents | Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized cost	6,001
Unrealized Gain	5
Unrealized Loss	0
Estimated Fair Value	6,006
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	821,730	881,779
Unrealized Gain	5,964	1,205
Unrealized Loss	(138)	(8)
Estimated Fair Value	$ 827,556	$ 882,976