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Commitments and Contingencies - Additional Information (Details) - USD ($) | Mar. 31, 2025 | Dec. 31, 2024
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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
Name: | fbiz_SBALoansProbabilityofFutureLosses
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