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Commitments and Contingencies SBA Recourse Reserve Rollforward (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Commitments and Contingencies Disclosure [Abstract] |  |  |  |
Beginning balance | $ 752 | $ 673 | $ 441 | $ 635
SBA recourse provision | 242 | 96 | 565 | 134
SBA Loan Charge Offs, Net | (11) | (6) | (23) | (6)
Ending balance | $ 983 | $ 763 | $ 983 | $ 763

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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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- 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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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration