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, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Net Income per Common Share
Net income (in dollars)	$ 95,387	$ 94,025	$ 118,549	$ 113,987
Basic net income per share:
Basic common shares	47,768	49,231	48,333	49,176
Basic net income per share (in dollars per share)	$ 2.00	$ 1.91	$ 2.45	$ 2.32
Diluted net income per share:
Basic common shares	47,768	49,231	48,333	49,176
Diluted restricted stock awards and stock options	588	1,263	723	1,169
Diluted common shares	48,356	50,494	49,056	50,345
Diluted net income per share (in dollars per share)	$ 1.97	$ 1.86	$ 2.42	$ 2.26