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Marketable Securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Apr. 30, 2014
Apr. 30, 2013
Schedule of Available-for-sale Securities [Line Items]    
Proceeds from sale and maturities of available-for-sale securities $ 485,375 $ 155,801
Amortization of discounts and premiums, net 2,607 2,192
Other Expense Income Net
   
Schedule of Available-for-sale Securities [Line Items]    
Net realized gain (loss) $ 70 $ (215)