Derivative Instruments - Additional Information (Detail) (USD $)	6 Months Ended
	Jul. 31, 2012	Jul. 31, 2011
Derivative [Line Items]
Net foreign currency exchange gain (loss)	$ 300,000	$ (4,100,000)
Average notional amounts of derivative financial instruments outstanding	$ 1,600,000,000	$ 1,500,000,000
Average maturities of derivatives, days	28 days	37 days