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		9 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017
Net Income per Common Share
Net income (in dollars)	$ 94,025	$ 81,625	$ 113,987	$ 62,666
Basic net income per share:
Basic common shares	49,231	48,846	49,176	48,729
Basic net income per share (in dollars per share)	$ 1.91	$ 1.67	$ 2.32	$ 1.29
Diluted net income per share:
Basic common shares	49,231	48,846	49,176	48,729
Diluted restricted stock awards and stock options	1,263	682	1,169	681
Diluted common shares	50,494	49,528	50,345	49,410
Diluted net income per share (in dollars per share)	$ 1.86	$ 1.65	$ 2.26	$ 1.27