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Commitments and Contingencies (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022
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Commitments to extend credit, primarily commercial loans |  |
Lending Related Commitments by Type [Line Items] |  |
Lending related commitments | $ 1,198,031 | $ 913,042
Standby letters of credit |  |
Lending Related Commitments by Type [Line Items] |  |
Lending related commitments | $ 17,938 | $ 15,013

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- Definition Lending related commitments. + References No definition available. + Details
Name: | fbiz_LendingRelatedCommitments
Namespace Prefix: | fbiz_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition [Line Items] for Lending Related Commitments by Type [Table] + References No definition available. + Details
Name: | fbiz_LendingRelatedCommitmentsByTypeLineItems
Namespace Prefix: | fbiz_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | fbiz_LendingRelatedCommitmentsByTypeAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | fbiz_LendingRelatedCommitmentsByTypeAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |