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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jul. 31, 2016
Jul. 31, 2015
Jul. 31, 2016
Jul. 31, 2015
Schedule of Available-for-sale Securities [Line Items]        
Proceeds from sales and maturities of available-for-sale securities     $ 109,357 $ 213,930
Amortization of discounts and premiums, net $ 567 $ 1,042 1,161 2,158
Other Income ( Expense ), Net        
Schedule of Available-for-sale Securities [Line Items]        
Net realized gain $ 9 $ 39 $ 22 $ 42