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Fair Value Measurements - Summary of Carrying Value and Fair Value of Debt Obligations (Parenthetical) (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Fair Value Disclosures [Abstract]
Secured Borrowings	$ 12,816	$ 13,674

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

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- References

No definition available.

+ Details

Name:	us-gaap_FairValueDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration