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Commitments and Contingencies SBA Recourse Reserve Rollforward (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2023 | Mar. 31, 2022
Commitments and Contingencies Disclosure [Abstract] |  |
Beginning balance | $ 441 | $ 635
SBA recourse provision | (18) | (76)
Ending balance | $ 423 | $ 559

X
- 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

X
- 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

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