Loans, Lease Receivables, and Allowance for Credit Losses - Loan and Lease Receivables Composition (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	$ 3,336,673	$ 3,113,850
Allowance for loan losses	36,690	35,785	$ 33,688
Deferred loan fees and costs, net	1,717	722
Loans and leases receivable, net	3,298,266	3,077,343
Commercial Real Estate
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	2,027,239	1,917,130
Commercial Real Estate \| Owner Occupied
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	287,005	273,397
Allowance for loan losses	1,837	1,615	1,588
Commercial Real Estate \| Non-owner Occupied
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	871,807	845,298
Allowance for loan losses	6,689	5,843	6,048
Commercial Real Estate \| Construction
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	236,590	221,086
Allowance for loan losses	1,930	2,022	2,518
Commercial Real Estate \| Multi-family
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	565,102	530,853
Allowance for loan losses	6,103	4,597	3,710
Commercial Real Estate \| 1-4 Family
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	66,735	46,496
Commercial and Industrial
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	1,264,111	1,151,720
Allowance for loan losses	19,282	20,934	19,010
Consumer and Other
Loans and Leases Receivable Disclosure
Total gross loans and leases receivable	45,323	45,000
Allowance for loan losses	$ 322	$ 282	$ 375

Financing receivable deferred income expenses.

No definition available.

Name:	fbiz_FinancingReceivableDeferredIncomeExpenses
Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansAndLeasesReceivableDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbiz_OwnerOccupiedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbiz_NonOwnerOccupiedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbiz_MultiFamilyLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbiz_To4FamilyLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fbiz_CommercialandIndustrialLoansandLeasesReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fbiz_ConsumerAndOtherLoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: