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Nature of Operations and Summary of Significant Accounting Policies Bank-Owned Life Insurance (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |
Bank owned life insurance death benefits | $ 133.7 | $ 133.8
Borrowings against cash surrender value of bank owned life insurance | $ 0.0 | $ 0.0

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- Definition Bank Owned Life Insurance Death Benefits + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Borrowings Against Cash Surrender Value of Bank Owned Life Insurance + References No definition available. + Details
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Namespace Prefix: | fbiz_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration