Bank Revolving Credit Facility (Details Textual) (USD $)	6 Months Ended
	Jul. 31, 2012	Jan. 31, 2012
Line of Credit Facility [Line Items]
Maximum borrowings	$ 450,000,000	$ 450,000,000
Debt instrument floor rate	1.00%
Term of credit facility extension option (in months)	12 months
Amended Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowings	450,000,000
Credit agreement maturity date	March 30, 2014 and provides for one, 12-month extension option
Interest rate of borrowings	LIBOR, subject to a floor of 100 basis points, plus 3.75%
Debt instrument variable rate basis	LIBOR
Debt instrument variable rate basis spread	3.75%
Amount of available borrowing that can be used for letters of credit or surety bonds	$ 100,000,000
Number of credit facility extension options	1