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, 2018	Jan. 31, 2018
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	$ 350,351	$ 101,967
Unrealized Gain	0	0
Unrealized Loss	(246)	(202)
Estimated Fair Value	350,105	101,765
Total Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	513,972	192,737
Unrealized Gain	0	0
Unrealized Loss	(246)	(202)
Estimated Fair Value	513,726	192,535
Commercial paper
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	99,039	15,946
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	99,039	15,946
U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	211,344	61,896
Unrealized Gain	0	0
Unrealized Loss	(188)	(158)
Estimated Fair Value	211,156	61,738
Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	39,968	24,125
Unrealized Gain	0	0
Unrealized Loss	(58)	(44)
Estimated Fair Value	39,910	24,081
Cash Equivalents
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	163,621	90,770
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	163,621	90,770
Cash Equivalents | Money market funds
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	158,639	90,770
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	158,639	$ 90,770
Cash Equivalents | Commercial paper
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	4,982
Unrealized Gain	0
Unrealized Loss	0
Estimated Fair Value	$ 4,982