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Financial Assets and Liabilities - Short-term investments (Details) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2015
Cash equivalents  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost $ 397,945us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized losses less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Estimated fair value 397,945us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember		 [1]
Cash equivalents | Money market funds  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 394,546us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MoneyMarketFundsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MoneyMarketFundsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized losses less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MoneyMarketFundsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MoneyMarketFundsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Estimated fair value 394,546us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MoneyMarketFundsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember		 [1]
Cash equivalents | Commercial paper  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 3,399us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized losses less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember
Estimated fair value 3,399us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashEquivalentsMember		 [1]
Short-term investments  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 136,549us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 43us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months (13)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value 136,579us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
Short-term investments | Certificates of deposit  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 5,950us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value 5,950us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
Short-term investments | Commercial paper  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 5,893us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value 5,893us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CommercialPaperMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
Short-term investments | U.S. government agency securities  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 24,907us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 6us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value 24,912us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
Short-term investments | Municipal bonds  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 1,404us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value 1,406us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
Short-term investments | Corporate debt securities  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 67,829us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 25us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months (9)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value 67,845us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
Short-term investments | Asset-backed securities  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale securities, cost 30,566us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized gains 10us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses less than 12 months (3)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Gross unrealized losses 12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Estimated fair value $ 30,573us-gaap_AvailableForSaleSecurities
/ us-gaap_CashAndCashEquivalentsAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 [1]
[1] See Note 5. Fair Value Measures for further discussion on fair values of cash equivalents and short-term investments.