v2.4.1.9

Stockholders' Equity , Performance Stock Units (Details) (Performance Shares [Member], USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Performance Share, Weighted Average Assumptions [Abstract]
Expected term in years	2 years 11 months 19 days	2 years 11 months 19 days
Volatility	38.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	48.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
Risk-free interest rate	1.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Dividend rate	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Granted	206us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod	25us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Granted, weighted-average grant-date fair value per share (in dollars per share)	$ 34.86us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	$ 22.44us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Compensation cost not yet recognized	$ 9.5us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Compensation cost not yet recognized period of recognition	2 years 6 months 18 days
Market-based [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share target range, percent	0.00%ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsSettlementRange / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheOneMember
Performance period	2 years
Market-based [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share target range, percent	150.00%ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsSettlementRange / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheOneMember
Performance period	3 years
Performance-based [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period	1 year
Performance-based [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share target range, percent	50.00%ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsSettlementRange / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Performance-based [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share target range, percent	150.00%ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsSettlementRange / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheTwoMember

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Settlement Range

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Unrecognized cost of unvested share-based compensation awards.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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