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Commitments and Contingencies SBA Recourse Reserve Rollforward (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract] |  |
Beginning balance | $ 2,849 | $ 1,750
SBA Recourse Provision | (295) | 6
SBA Loan Charge Offs, Net | (38) | (91)
Ending balance | $ 2,516 | $ 1,665

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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 (Benefit) Provision + References No definition available. + Details
Name: | fbiz_SBARecourseBenefitProvision
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