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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

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