Loans, Lease Receivables, and Allowance for Credit Losses - Allowance for Credit Losses and Balances by Type of Allowance Methodology (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	$ 26,867	$ 25,286
Allowance for credit losses, individually evaluated for impairment	8,918	5,989
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	35,785	31,275	$ 24,230	$ 24,336
Loans and lease receivables, collectively evaluated for impairment	3,085,483	2,829,421
Loans and lease receivables, individually evaluated for impairment	28,367	20,597
Loans and Leases Receivable, before Fees, Gross	3,113,850	2,850,018
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest			12,560	15,110
Loans and Leases Receivable, before Fees, Gross	1,917,130	1,699,871
Commercial Real Estate \| Owner Occupied
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	1,615	1,525
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	1,615	1,525
Loans and lease receivables, collectively evaluated for impairment	272,806	256,479
Loans and lease receivables, individually evaluated for impairment	591	0
Loans and Leases Receivable, before Fees, Gross	273,397	256,479
Commercial Real Estate \| Non-owner Occupied
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	5,843	5,596
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	5,843	5,596
Loans and lease receivables, collectively evaluated for impairment	845,298	773,494
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	845,298	773,494
Commercial Real Estate \| Construction
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	2,022	1,244
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	2,022	1,244
Loans and lease receivables, collectively evaluated for impairment	221,086	193,080
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	221,086	193,080
Commercial Real Estate \| Multi-family
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	4,597	3,562
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	4,597	3,562
Loans and lease receivables, collectively evaluated for impairment	530,853	450,529
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	530,853	450,529
Commercial Real Estate \| 1-4 Family
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	492	221
Allowance for credit losses, individually evaluated for impairment	0	22
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	492	243
Loans and lease receivables, collectively evaluated for impairment	46,496	26,267
Loans and lease receivables, individually evaluated for impairment	0	22
Loans and Leases Receivable, before Fees, Gross	46,496	26,289
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	12,016	12,743
Allowance for credit losses, individually evaluated for impairment	8,918	5,967
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	20,934	18,710	11,128	8,413
Loans and lease receivables, collectively evaluated for impairment	1,123,944	1,085,260
Loans and lease receivables, individually evaluated for impairment	27,776	20,575
Loans and Leases Receivable, before Fees, Gross	1,151,720	1,105,835
Consumer and Other
Financing Receivable, Allowance for Credit Losses
Allowance for credit losses, collectively evaluated for impairment	282	395
Allowance for credit losses, individually evaluated for impairment	0	0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	282	395	$ 542	$ 813
Loans and lease receivables, collectively evaluated for impairment	45,000	44,312
Loans and lease receivables, individually evaluated for impairment	0	0
Loans and Leases Receivable, before Fees, Gross	$ 45,000	$ 44,312

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
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ 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: