Fair Value Measurements and Marketable Securities - Redeemable convertible preferred stock warrant liability (Details) - Level 3 - Redeemable convertible preferred stock warrant - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018
Summary of changes in the redeemable convertible preferred stock warrant liability
Balance at beginning of period	$ 5,704	$ 961	$ 4,537	$ 961
Adjustment resulting from change in fair value recognized in the condensed consolidated statements of operations	4,855	2,055	6,022	2,055
Reclassification of liability for redeemable convertible preferred stock warrants to additional paid-in capital upon initial public offering	$ (10,559)		$ (10,559)
Balance at end of period		$ 3,016		$ 3,016
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	Other Nonoperating Income (Expense)	Other Nonoperating Income (Expense)	Other Nonoperating Income (Expense)	Other Nonoperating Income (Expense)