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Commitments and Contingencies Commitments and Contingencies (SBA Recourse Reserve) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract] |  |
Beginning Balance | $ 1,345 | $ 2,956
SBA recourse provision | (278) | 188
SBA Loan Charge Offs, Net | (344) | (1,799)
Ending Balance | $ 723 | $ 1,345

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