Fair Value Measurements (Schedule of Financial Assets Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified
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Mar. 31, 2015
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Sep. 30, 2014
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Cash equivalents, fair value |
$ 72,629us-gaap_CashAndCashEquivalentsFairValueDisclosure |
$ 43,618us-gaap_CashAndCashEquivalentsFairValueDisclosure |
Investments, fair value |
766,564us-gaap_AvailableForSaleSecurities |
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Financial assets measured at fair value on a recurring basis, total |
839,193us-gaap_AssetsFairValueDisclosureRecurring |
890,412us-gaap_AssetsFairValueDisclosureRecurring |
Quoted Prices In Active Markets For Identical Securities (Level 1) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Cash equivalents, fair value |
72,629us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
43,618us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Financial assets measured at fair value on a recurring basis, total |
72,629us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
43,618us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Financial assets measured at fair value on a recurring basis, total |
766,564us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
846,794us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Short-Term Investments [Member] | Certificates of Deposit [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
6,653us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember |
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Short-Term Investments [Member] | Certificates of Deposit [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
6,653us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember |
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Short-Term Investments [Member] | Corporate Bonds and Notes [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
169,864us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
205,698us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
Short-Term Investments [Member] | Corporate Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
169,864us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
205,698us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
Short-Term Investments [Member] | Municipal Bonds and Notes [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
44,975us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
43,430us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
Short-Term Investments [Member] | Municipal Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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|
Investments, fair value |
44,975us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
43,430us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
Short-Term Investments [Member] | U.S. Government Securities [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
5,001us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
5,006us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
Short-Term Investments [Member] | U.S. Government Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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|
Investments, fair value |
5,001us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
5,006us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
Short-Term Investments [Member] | U.S. Government Agency Securities [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
72,369us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
109,743us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Short-Term Investments [Member] | U.S. Government Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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|
Investments, fair value |
72,369us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
109,743us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Long-Term Investments [Member] | Corporate Bonds and Notes [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
294,974us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
325,282us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
Long-Term Investments [Member] | Corporate Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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|
Investments, fair value |
294,974us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
325,282us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember |
Long-Term Investments [Member] | Municipal Bonds and Notes [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
61,936us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
24,582us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
Long-Term Investments [Member] | Municipal Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
|
|
Investments, fair value |
61,936us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
24,582us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = ffiv_MunicipalBondsAndNotesMember |
Long-Term Investments [Member] | U.S. Government Securities [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
7,412us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
7,407us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
Long-Term Investments [Member] | U.S. Government Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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|
Investments, fair value |
7,412us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
7,407us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentDebtSecuritiesMember |
Long-Term Investments [Member] | U.S. Government Agency Securities [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
100,556us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
123,087us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Long-Term Investments [Member] | U.S. Government Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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|
Investments, fair value |
100,556us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
123,087us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Long-Term Investments [Member] | Foreign Government Debt Securities [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
2,824us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
2,559us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
Long-Term Investments [Member] | Foreign Government Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |
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Investments, fair value |
$ 2,824us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |
$ 2,559us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = ffiv_LongTermInvestmentsMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember |