Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Oct. 31, 2017	Jan. 31, 2017
Financial Liability:
Redeemable convertible preferred stock warrant liability	$ 0	$ 1,272
Recurring
Short-term investments:
Total financial assets	271,566	102,299
Financial Liability:
Redeemable convertible preferred stock warrant liability		1,272
Total financial liability		1,272
Recurring | Level 1
Short-term investments:
Total financial assets	271,566	102,299
Financial Liability:
Redeemable convertible preferred stock warrant liability		0
Total financial liability		0
Recurring | Level 2
Short-term investments:
Total financial assets	0	0
Financial Liability:
Redeemable convertible preferred stock warrant liability		0
Total financial liability		0
Recurring | Level 3
Short-term investments:
Total financial assets	0	0
Financial Liability:
Redeemable convertible preferred stock warrant liability		1,272
Total financial liability		1,272
U.S. government treasury securities | Recurring
Short-term investments:
Short-term investments:	45,810	47,195
U.S. government treasury securities | Recurring | Level 1
Short-term investments:
Short-term investments:	45,810	47,195
U.S. government treasury securities | Recurring | Level 2
Short-term investments:
Short-term investments:	0	0
U.S. government treasury securities | Recurring | Level 3
Short-term investments:
Short-term investments:	0	0
Money Market Funds | Recurring
Cash and cash equivalents:
Cash and cash equivalents:	225,756	35,104
Money Market Funds | Recurring | Level 1
Cash and cash equivalents:
Cash and cash equivalents:	225,756	35,104
Money Market Funds | Recurring | Level 2
Cash and cash equivalents:
Cash and cash equivalents:	0	0
Money Market Funds | Recurring | Level 3
Cash and cash equivalents:
Cash and cash equivalents:	$ 0	0
U.S. government treasury securities | Recurring
Cash and cash equivalents:
Cash and cash equivalents:		20,000
U.S. government treasury securities | Recurring | Level 1
Cash and cash equivalents:
Cash and cash equivalents:		$ 20,000