Net Income per Common Share - Reconciliation between basic and diluted 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
Net Income per Common Share
Net income (in dollars)	$ 11,119	$ 10,077	$ 23,162	$ 19,962
Basic net income per share:
Basic common shares	48,450	49,169	48,619	49,148
Basic net income per share (in dollars per share)	$ 0.23	$ 0.20	$ 0.48	$ 0.41
Diluted net income per share:
Basic common shares	48,450	49,169	48,619	49,148
Diluted restricted stock awards and stock options	666	1,246	817	1,124
Diluted common shares	49,116	50,415	49,436	50,272
Diluted net income per share (in dollars per share)	$ 0.23	$ 0.20	$ 0.47	$ 0.40