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Commitments and Contingencies - Additional Information (Details) - USD ($)	Mar. 31, 2025	Dec. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
SBA loans, probability of future losses	$ 566,000	$ 645,000

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- Definition

SBA Loans, Probability of Future Losses

+ References

No definition available.

+ Details

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Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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