Net Income (Loss) 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, 2018	Jul. 31, 2017	Jul. 31, 2018	Jul. 31, 2017
Earnings per share
Net income (loss) (in dollars)	$ 10,077	$ (8,568)	$ 19,962	$ (18,959)
Basic net income (loss) per share:
Basic common shares	49,169	48,689	49,148	48,669
Basic net income (loss) per share (in dollars per share)	$ 0.20	$ (0.18)	$ 0.41	$ (0.39)
Diluted net income (loss) per share:
Basic common shares	49,169	48,689	49,148	48,669
Diluted restricted stock awards and stock options	1,246	0	1,124	0
Diluted common shares	50,415	48,689	50,272	48,669
Diluted net income (loss) per share (in dollars per share)	$ 0.20	$ (0.18)	$ 0.40	$ (0.39)