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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 ($)
Debt Instrument [Line Items]
Debt instrument issued amount	$ 8,000
Debt instrument interim interest rate	6.047%

X

- Definition

Debt instrument interim interest rate.

+ References

No definition available.

+ Details

Name:	fdus_DebtInstrumentInterimInterestRate
Namespace Prefix:	fdus_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

- 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

X

- 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

X

- Details

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

X

- Details

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

X

- Details

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