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Commitments and Contingencies - SBA Recourse Reserve (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Commitments and Contingencies Disclosure [Abstract] |  |  |  |
Beginning balance | $ 507 | $ 906 | $ 645 | $ 955
SBA recourse (benefit) expense | (5) | 466 | (64) | 583
Charge-offs, net | (0) | (40) | (79) | (206)
Ending balance | $ 502 | $ 1,332 | $ 502 | $ 1,332

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- Definition SBA Recourse Provision (Benefit) + References No definition available. + Details
Name: | fbiz_SBARecourseProvisionBenefit
Namespace Prefix: | fbiz_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition SBA Recourse Reserve + References No definition available. + Details
Name: | fbiz_SBARecourseReserve
Namespace Prefix: | fbiz_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition SBA Loan Charge Offs, Net + References No definition available. + Details
Name: | fbiz_SbaLoanChargeOffsNet
Namespace Prefix: | fbiz_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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