Employee benefits (Details) - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Jan. 31, 2020 |
Jan. 31, 2019 |
Jan. 31, 2018 |
|
| Defined Contribution Plan Disclosure [Line Items] | |||
| Maximum coverage per incident under self-insurance | $ 200,000 | ||
| Accrued compensation | |||
| Defined Contribution Plan Disclosure [Line Items] | |||
| Liability for self-insured medical claims | $ 3,700,000 | $ 1,400,000 | |
| Supplemental Employee Retirement Plan | 401(k) | |||
| Defined Contribution Plan Disclosure [Line Items] | |||
| Percent of employees eligible earnings | 3.50% | ||
| Employer matching contribution expense | $ 3,700,000 | $ 1,800,000 | $ 1,400,000 |