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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Schedule of Available-for-sale Securities [Line Items]    
Proceeds from sales and maturities of available-for-sale securities $ 64,903 $ 57,219
Amortization of discounts and premiums, net 803 594
Other Income ( Expense ), Net    
Schedule of Available-for-sale Securities [Line Items]    
Net realized gain (loss) $ 14 $ 13