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Employee Benefit Plans - Bank Owned Life Insurance - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Bank Owned Life Insurance [Line Items]
Bank-owned life insurance	$ 57,210	$ 55,536
Bank owned life insurance death benefits	133,800	133,700
Insured executive officers with deferred compensation plans
Bank Owned Life Insurance [Line Items]
Bank-owned life insurance	3,200	3,100
Bank owned life insurance death benefits	6,200	6,200
Other insured individuals
Bank Owned Life Insurance [Line Items]
Bank-owned life insurance	$ 54,000	$ 52,400

X

- Definition

Bank owned life insurance death benefits.

+ References

No definition available.

+ Details

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Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Bank owned life insurance.

+ References

No definition available.

+ Details

Name:	fbiz_BankOwnedLifeInsuranceLineItems
Namespace Prefix:	fbiz_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The carrying amount of a life insurance policy on an officer, executive or employee for which the reporting entity (a bank) is entitled to proceeds from the policy upon death of the insured or surrender of the insurance policy.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	fbiz_BankOwnedLifeInsuranceInsuredAxis=fbiz_InsuredExecutiveOfficersWithDeferredCompensationPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	fbiz_BankOwnedLifeInsuranceInsuredAxis=fbiz_OtherInsuredIndividualsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: