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Debt - Summary of Issued and Outstanding SBA Debentures (Parenthetical) (Details) - SBA Debentures - Forecast - 6.047% Fixed Interest Rate $ in Thousands | Sep. 30, 2023 USD ($)
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Debt Instrument [Line Items] |
Debt instrument issued amount | $ 8,000
Debt instrument interim interest rate | 6.047%

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- Definition Debt instrument interim interest rate. + References No definition available. + Details
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Namespace Prefix: | fdus_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Debt instrument issued amount. + References No definition available. + Details
Name: | fdus_DebtInstrumentIssuedAmount
Namespace Prefix: | fdus_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LongtermDebtTypeAxis=fdus_SmallBusinessAdministrationDebenturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=fdus_SixPointZeroFourSevenPercentInterestRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |