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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Apr. 30, 2016
Apr. 30, 2015
Schedule of Available-for-sale Securities [Line Items]    
Proceeds from sales and maturities of available-for-sale securities $ 57,219 $ 80,942
Amortization of discounts and premiums, net 594 1,116
Other Expense, Net    
Schedule of Available-for-sale Securities [Line Items]    
Net realized gain $ 13 $ 3