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Financial Instruments and Fair Value , Additional Information (Details) $ in Millions | Mar. 31, 2020 USD ($)
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Financial Instruments and Fair Value [Abstract] |
Accrued interest on available-for-sale securities | $ 4.7

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- Definition Debt Securities, Available-For-Sale, Accrued Interest, After Allowance For Credit Loss + References No definition available. + Details
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Namespace Prefix: | ftnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Financial Instruments and Fair Value [Abstract] + References No definition available. + Details
Name: | ftnt_FinancialInstrumentsAndFairValueAbstract
Namespace Prefix: | ftnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration