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Financial Instruments and Fair Value , Additional Information (Details) $ in Millions	Mar. 31, 2020 USD ($)
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

X

- 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