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	Jul. 31, 2018	Jan. 31, 2018
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	$ 343,624	$ 101,967
Unrealized Gain	6	0
Unrealized Loss	(256)	(202)
Estimated Fair Value	343,374	101,765
Total Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	511,208	192,737
Unrealized Gain	6	0
Unrealized Loss	(256)	(202)
Estimated Fair Value	510,958	192,535
Commercial paper
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	56,191	15,946
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	56,191	15,946
U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	248,081	61,896
Unrealized Gain	0	0
Unrealized Loss	(217)	(158)
Estimated Fair Value	247,864	61,738
Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	39,352	24,125
Unrealized Gain	6	0
Unrealized Loss	(39)	(44)
Estimated Fair Value	39,319	24,081
Cash Equivalents
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	167,584	90,770
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	167,584	90,770
Cash Equivalents | Money market funds
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	155,347	90,770
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	155,347	$ 90,770
Cash Equivalents | Commercial paper
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	12,237
Unrealized Gain	0
Unrealized Loss	0
Estimated Fair Value	$ 12,237