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Commitments and Contingencies - Additional Information (Details) - USD ($)	Dec. 31, 2024	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Accrued credit losses for financial instruments with off-balance sheet risk	$ 0	$ 0
SBA loans, probability of future losses	$ 645,000,000	$ 955,000,000

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- Definition

Accrued credit losses for financial instruments with off balance sheet risk.

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Period Type:	instant

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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.

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Namespace Prefix:	us-gaap_
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Balance Type:	na
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