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Commitments and Contingencies (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Commitments to extend credit, primarily commercial loans
Lending Related Commitments by Type [Line Items]
Lending related commitments	$ 654,093	$ 555,374
Standby letters of credit
Lending Related Commitments by Type [Line Items]
Lending related commitments	$ 8,086	$ 8,918

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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: