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Commitments and Contingencies SBA Recourse Reserve Rollforward (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract] |  |
Beginning balance | $ 1,345 | $ 2,956
SBA recourse provision | 25 | 481
SBA Loan Charge Offs, Net | (284) | (161)
Ending balance | $ 1,086 | $ 3,276

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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