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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jul. 31, 2015
Jul. 31, 2014
Jul. 31, 2015
Jul. 31, 2014
Schedule of Available-for-sale Securities [Line Items]        
Proceeds from sales and maturities of available-for-sale securities     $ 213,930 $ 631,950
Amortization of discounts and premiums, net $ 1,042 $ 1,499 2,158 4,106
Other (Expense) Income, Net        
Schedule of Available-for-sale Securities [Line Items]        
Net realized gains $ 39 $ 101 $ 42 $ 171