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Debt - Summary of Outstanding Debt Net of Unamortized Deferred Financing Costs (Parenthetical) (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
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Debt Disclosure [Abstract] |  |
Secured Borrowings | $ 16,319 | $ 16,880

X
- Definition Secured borrowings. + References No definition available. + Details
Name: | fdus_SecuredBorrowings
Namespace Prefix: | fdus_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration