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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Oct. 31, 2016
Oct. 31, 2015
Oct. 31, 2016
Oct. 31, 2015
Schedule of Available-for-sale Securities [Line Items]        
Proceeds from sales and maturities of available-for-sale securities     $ 218,400 $ 296,172
Amortization of discounts and premiums, net $ 550 $ 997 1,711 3,155
Other Income ( Expense ), Net        
Schedule of Available-for-sale Securities [Line Items]        
Net realized gain (loss) $ (96) $ 11 $ (74) $ 53