Loans, Lease Receivables, and Allowance for Credit Losses - Allowance for Credit Losses and Balances by Type of Allowance Methodology (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	$ 31,065	$ 26,867
Allowance for credit losses, individually evaluated for impairment	5,625	8,918
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	36,690	35,785	$ 33,688
Loans and lease receivables, collectively evaluated for impairment	3,313,160	3,085,483
Loans and lease receivables, individually evaluated for impairment	23,513	28,367
Loans and Leases Receivable, before Fees, Gross	3,336,673	3,113,850
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	2,027,239	1,917,130
Commercial Real Estate \| Owner Occupied
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	1,837	1,615
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	1,837	1,615	1,588
Loans and lease receivables, collectively evaluated for impairment	287,005	272,806
Loans and lease receivables, individually evaluated for impairment	0	591
Loans and Leases Receivable, before Fees, Gross	287,005	273,397
Commercial Real Estate \| Non-owner Occupied
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	6,689	5,843
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	6,689	5,843	6,048
Loans and lease receivables, collectively evaluated for impairment	871,807	845,298
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	871,807	845,298
Commercial Real Estate \| Construction
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	1,930	2,022
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	1,930	2,022	2,518
Loans and lease receivables, collectively evaluated for impairment	236,590	221,086
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	236,590	221,086
Commercial Real Estate \| Multi-family
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	6,103	4,597
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	6,103	4,597	3,710
Loans and lease receivables, collectively evaluated for impairment	565,102	530,853
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	565,102	530,853
Commercial Real Estate \| 1-4 Family
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	527	492
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	527	492	439
Loans and lease receivables, collectively evaluated for impairment	66,735	46,496
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	66,735	46,496
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	13,657	12,016
Allowance for credit losses, individually evaluated for impairment	5,625	8,918
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	19,282	20,934	19,010
Loans and lease receivables, collectively evaluated for impairment	1,240,598	1,123,944
Loans and lease receivables, individually evaluated for impairment	23,513	27,776
Loans and Leases Receivable, before Fees, Gross	1,264,111	1,151,720
Consumer and Other
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	322	282
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	322	282	$ 375
Loans and lease receivables, collectively evaluated for impairment	45,323	45,000
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	$ 45,323	$ 45,000

Financing receivable allowance for credit losses collectively evaluated for impairments.

No definition available.

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

- Definition

Financing receivable allowance for credit losses individually evaluated for impairment.

+ References

No definition available.

+ Details

Name:	fbiz_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment
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

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

+ Details

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

- Definition

Amount of financing receivable collectively evaluated for credit loss. Excludes financing receivable individually evaluated for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.5.a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11C
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11C

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
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

Amount of financing receivable individually evaluated for credit loss. Excludes financing receivable collectively evaluated for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.5.a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11C
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11C

+ Details

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

- 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_OneToFourFamilyLoansMember
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: