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, 2018	Jan. 31, 2018
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	$ 335,611	$ 101,967
Unrealized Gain	1	0
Unrealized Loss	(327)	(202)
Estimated Fair Value	335,285	101,765
Total Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	508,506	192,737
Unrealized Gain	1	0
Unrealized Loss	(328)	(202)
Estimated Fair Value	508,179	192,535
Commercial paper
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	43,639	15,946
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	43,639	15,946
U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	251,000	61,896
Unrealized Gain	0	0
Unrealized Loss	(270)	(158)
Estimated Fair Value	250,730	61,738
Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	40,972	24,125
Unrealized Gain	1	0
Unrealized Loss	(57)	(44)
Estimated Fair Value	40,916	24,081
Cash Equivalents
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	172,895	90,770
Unrealized Gain	0	0
Unrealized Loss	(1)	0
Estimated Fair Value	172,894	90,770
Cash Equivalents | Money market funds
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	145,932	90,770
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	145,932	$ 90,770
Cash Equivalents | Commercial paper
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	9,972
Unrealized Gain	0
Unrealized Loss	0
Estimated Fair Value	9,972
Cash Equivalents | U.S. treasury securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	9,995
Unrealized Gain	0
Unrealized Loss	0
Estimated Fair Value	9,995
Cash Equivalents | Corporate debt securities
Cash Equivalents and Short-term Investments [Abstract]
Amortized Cost	6,996
Unrealized Gain	0
Unrealized Loss	(1)
Estimated Fair Value	$ 6,995