Derivative Instruments - Additional Information (Detail) (USD $)	9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011
Derivative [Line Items]
Net foreign currency exchange gain (loss)	$ 9,800,000	$ 10,000,000
Average notional amounts of derivative financial instruments outstanding	$ 1,800,000,000	$ 1,400,000,000
Average maturities of derivatives, days	28 days	22 days