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, 2019	Jan. 31, 2019
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	$ 471,499	$ 516,231
Unrealized Gain	392	50
Unrealized Loss	(26)	(73)
Estimated Fair Value	471,865	516,208
U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	204,210	195,913
Unrealized Gain	189	37
Unrealized Loss	(22)	(53)
Estimated Fair Value	204,377	195,897
Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	146,135	69,483
Unrealized Gain	203	13
Unrealized Loss	(3)	(19)
Estimated Fair Value	146,335	69,477
Cash Equivalents
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	121,154	250,835
Unrealized Gain	0	0
Unrealized Loss	(1)	(1)
Estimated Fair Value	121,153	250,834
Cash Equivalents | Money market funds
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	116,155	247,426
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	116,155	247,426
Cash Equivalents | Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	4,999	3,409
Unrealized Gain	0	0
Unrealized Loss	(1)	(1)
Estimated Fair Value	4,998	3,408
Short-term Investments [Member]
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	350,345	265,396
Unrealized Gain	392	50
Unrealized Loss	(25)	(72)
Estimated Fair Value	$ 350,712	$ 265,374