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

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- 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 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 5 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26853-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
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