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Commitments and Contingencies SBA Recourse Reserve Rollforward (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract] |  |
Beginning balance | $ 723 | $ 1,345
SBA recourse provision | (130) | 25
SBA Loan Charge Offs, Net | 0 | (284)
Ending balance | $ 593 | $ 1,086

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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