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Fair Value Of Financial Instruments (Details) (USD $)
6 Months Ended
Jul. 31, 2011
Jul. 31, 2011
Derivatives Not Designated As Hedging Instruments [Member]
Jul. 31, 2010
Derivatives Not Designated As Hedging Instruments [Member]
Jul. 31, 2011
Convertible Senior Debentures [Member]
Jan. 31, 2011
Convertible Senior Debentures [Member]
Dec. 31, 2006
Convertible Senior Debentures [Member]
Net unrealized gain on foreign currency forward contracts   $ 5,500,000 $ 4,700,000      
Net unrealized loss on foreign currency forward contracts   13,600,000 12,400,000      
Convertible senior debentures, face value       350,000,000 350,000,000 350,000,000
Fair value of the convertible senior debentures $ 358,300,000