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Deposits Deposits Narrative (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Deposits [Abstract] |  |
Time Deposits, $250,000 or More | $ 28.7 | $ 63.7

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- Definition Time Deposits, $250,000 or More + References No definition available. + Details
Name: | fbiz_TimeDeposits250000orMore
Namespace Prefix: | fbiz_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DepositsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration