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Marketable Securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jul. 31, 2014
Jul. 31, 2013
Jul. 31, 2014
Jul. 31, 2013
Schedule of Available-for-sale Securities [Line Items]        
Proceeds from sale and maturities of available-for-sale securities     $ 631,950 $ 245,759
Amortization of discounts and premiums, net 1,499 2,527 4,106 4,719
Other (Expense) Income, Net
       
Schedule of Available-for-sale Securities [Line Items]        
Net realized gain (loss) $ 101 $ 12 $ 171 $ (202)