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Marketable Securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Schedule of Available-for-sale Securities [Line Items]    
Proceeds from sale and maturities of available-for-sale securities $ 80,942us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities $ 485,375us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities
Amortization of discounts and premiums, net 1,116us-gaap_AmortizationOfDebtDiscountPremium 2,607us-gaap_AmortizationOfDebtDiscountPremium
Other (Expense) Income, Net    
Schedule of Available-for-sale Securities [Line Items]    
Net realized gains $ 3us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember
$ 70us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember