Net income per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018
Numerator (basic and diluted):
Net income	$ 19,366	$ 22,517	$ 61,188	$ 45,094
Denominator (basic):
Weighted-average common shares outstanding (in shares)	64,220	61,880	63,289	61,531
Denominator (diluted):
Weighted-average common shares outstanding (in shares)	64,220	61,880	63,289	61,531
Weighted-average dilutive effect of stock options and restricted stock units (in shares)	1,363	1,517	1,496	1,529
Diluted weighted-average common shares outstanding (in shares)	65,583	63,397	64,785	63,060
Net income per share:
Basic (in usd per share)	$ 0.30	$ 0.36	$ 0.97	$ 0.73
Diluted (in usd per share)	$ 0.30	$ 0.36	$ 0.94	$ 0.72
Antidilutive securities excluded from computation of earnings per share (in shares)	300	100	300	100