Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jul. 31, 2018	Jan. 31, 2018
Assets:
Cash equivalents, fair value	$ 167,584	$ 90,770
Short term investments, fair value	343,374	101,765
Total cash equivalents and short-term investments	510,958	192,535
Level 1
Assets:
Cash equivalents, fair value	155,347	90,770
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	155,347	90,770
Level 2
Assets:
Cash equivalents, fair value	12,237	0
Short term investments, fair value	343,374	101,765
Total cash equivalents and short-term investments	355,611	101,765
Level 3
Assets:
Cash equivalents, fair value	0	0
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	0	0
Commercial paper
Assets:
Short term investments, fair value	56,191	15,946
Commercial paper | Level 1
Assets:
Short term investments, fair value	0	0
Commercial paper | Level 2
Assets:
Short term investments, fair value	56,191	15,946
Commercial paper | Level 3
Assets:
Short term investments, fair value	0	0
U.S. treasury securities
Assets:
Short term investments, fair value	247,864	61,738
U.S. treasury securities | Level 1
Assets:
Short term investments, fair value	0	0
U.S. treasury securities | Level 2
Assets:
Short term investments, fair value	247,864	61,738
U.S. treasury securities | Level 3
Assets:
Short term investments, fair value	0	0
Corporate debt securities
Assets:
Short term investments, fair value	39,319	24,081
Corporate debt securities | Level 1
Assets:
Short term investments, fair value	0	0
Corporate debt securities | Level 2
Assets:
Short term investments, fair value	39,319	24,081
Corporate debt securities | Level 3
Assets:
Short term investments, fair value	0	0
Money market funds
Assets:
Cash equivalents, fair value	155,347	90,770
Money market funds | Level 1
Assets:
Cash equivalents, fair value	155,347	90,770
Money market funds | Level 2
Assets:
Cash equivalents, fair value	0	0
Money market funds | Level 3
Assets:
Cash equivalents, fair value	0	$ 0
Commercial paper
Assets:
Cash equivalents, fair value	12,237
Commercial paper | Level 1
Assets:
Cash equivalents, fair value	0
Commercial paper | Level 2
Assets:
Cash equivalents, fair value	12,237
Commercial paper | Level 3
Assets:
Cash equivalents, fair value	$ 0