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Fair Value Measurements (Schedule of Financial Assets Measured at Fair Value on a Recurring Basis) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
Cash equivalents, fair value $ 72,629us-gaap_CashAndCashEquivalentsFairValueDisclosure $ 43,618us-gaap_CashAndCashEquivalentsFairValueDisclosure
Investments, fair value 766,564us-gaap_AvailableForSaleSecurities  
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
Investments, fair value 6,653us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_ShortTermInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
 
Short-Term Investments [Member] | Certificates of Deposit [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
Investments, fair value 6,653us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ invest_InvestmentAxis
= us-gaap_ShortTermInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
 
Short-Term Investments [Member] | Corporate Bonds and Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]    
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