Net Income per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Reconciliation between basic and diluted net income per share
Net income	$ 48,291	$ 43,555	$ 48,805	$ 44,600
Basic net income per share:
Basic common shares	20,053	19,845	19,971	19,804
Basic net income per share	$ 2.41	$ 2.19	$ 2.44	$ 2.25
Diluted net income per share:
Basic common shares	20,053	19,845	19,971	19,804
Stock options and restricted stock awards	348	327	338	405
Diluted common shares	20,401	20,172	20,309	20,209
Diluted net income per share	$ 2.37	$ 2.16	$ 2.40	$ 2.21