Income taxes - Narrative (Details) - USD ($)	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017
Operating Loss Carryforwards [Line Items]
Increase in valuation allowance	$ 5,400,000	$ 182,900,000
Unrecognized tax benefits	24,400,000	$ 18,600,000	$ 11,700,000	$ 9,600,000
Income tax benefit, favorable impact on effective tax rate	1,600,000
Income tax penalties and interest accrued	0
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	1,900,000,000
Research and development credits and other credits	56,900,000
State
Operating Loss Carryforwards [Line Items]
Research and development credits and other credits	46,700,000
State | California
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	528,700,000
State | Other State Board
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 682,100,000