Income taxes - Effective tax rate reconciliation (Details)	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax benefit at statutory rate	21.00%
State and local tax, net of federal benefit	2.80%
Permanent differences	(1.50%)
Equity compensation	6.80%
Foreign taxes	8.10%
Other	(4.30%)
Change in valuation allowance	(24.80%)
Effective income tax rate	8.10%	0.00%	0.00%