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Marketable Securities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Jan. 31, 2015 Securities	Jan. 31, 2014 Securities	Jan. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale and maturities of available-for-sale securities	$ 830,297us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	$ 451,866us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities	$ 207,576us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities
Amortization of discounts and premiums, net	6,696us-gaap_AmortizationOfDebtDiscountPremium	10,932us-gaap_AmortizationOfDebtDiscountPremium	5,276us-gaap_AmortizationOfDebtDiscountPremium
Number of securities with unrealized loss positions	172us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1	219us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Other (Expense) Income, Net
Schedule of Available-for-sale Securities [Line Items]
Net realized gain	$ 237us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember	$ 101us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember	$ 248us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember

X

- Definition

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

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-Name Regulation S-X (SX)

-Number 210

-Section 03

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-Article 5

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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The cash inflow associated with the sale or maturity (principal being due) of securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

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