Income taxes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018	Jan. 31, 2019
Income Tax Disclosure [Abstract]
Income tax provision (benefit)	$ 4,370,000	$ (1,029,000)	$ 13,826,000	$ (3,067,000)
Effective tax rate - (benefit) expense	18.40%	(4.80%)	18.40%	(7.30%)
Decrease in effective tax rate, primarily due to excess tax benefit on stock-based compensation expense	4.80%	27.00%	4.60%	29.50%
Gross unrecognized tax benefits	$ 2,000,000.0		$ 2,000,000.0		$ 1,700,000
Unrecognized tax benefit	400,000		400,000		$ 0
Unrecognized tax benefits that would impact the effective tax rate	$ 1,900,000		$ 1,900,000