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	Apr. 30, 2018	Jan. 31, 2018
Assets:
Cash equivalents, fair value	$ 172,894	$ 90,770
Short term investments, fair value	335,285	101,765
Total cash equivalents and short-term investments	508,179	192,535
Level 1
Assets:
Cash equivalents, fair value	145,932	90,770
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	145,932	90,770
Level 2
Assets:
Cash equivalents, fair value	26,962	0
Short term investments, fair value	335,285	101,765
Total cash equivalents and short-term investments	362,247	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	43,639	15,946
Commercial paper | Level 1
Assets:
Short term investments, fair value	0	0
Commercial paper | Level 2
Assets:
Short term investments, fair value	43,639	15,946
Commercial paper | Level 3
Assets:
Short term investments, fair value	0	0
U.S. treasury securities
Assets:
Short term investments, fair value	250,730	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	250,730	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	40,916	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	40,916	24,081
Corporate debt securities | Level 3
Assets:
Short term investments, fair value	0	0
Money market funds
Assets:
Cash equivalents, fair value	145,932	90,770
Money market funds | Level 1
Assets:
Cash equivalents, fair value	145,932	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	9,972
Commercial paper | Level 1
Assets:
Cash equivalents, fair value	0
Commercial paper | Level 2
Assets:
Cash equivalents, fair value	9,972
Commercial paper | Level 3
Assets:
Cash equivalents, fair value	0
U.S. treasury securities
Assets:
Cash equivalents, fair value	9,995
U.S. treasury securities | Level 1
Assets:
Cash equivalents, fair value	0
U.S. treasury securities | Level 2
Assets:
Cash equivalents, fair value	9,995
U.S. treasury securities | Level 3
Assets:
Cash equivalents, fair value	0
Corporate debt securities
Assets:
Cash equivalents, fair value	6,995
Corporate debt securities | Level 1
Assets:
Cash equivalents, fair value	0
Corporate debt securities | Level 2
Assets:
Cash equivalents, fair value	6,995
Corporate debt securities | Level 3
Assets:
Cash equivalents, fair value	$ 0