Securities - Summary of Unrecognized Loss for Securities Held-To-Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Fair Value
Fair Value Less than 12 months	$ 1,355	$ 959
Unrealized Loss Less than 12 months	2	1
Fair Value 12 months or longer	2,738	3,680
Unrealized Loss 12 months or longer	78	68
Total Fair Value	4,093	4,639
Total Unrealized Loss	80	69
Municipal Securities
Fair Value
Fair Value Less than 12 months	1,355	959
Unrealized Loss Less than 12 months	2	1
Fair Value 12 months or longer	0	680
Unrealized Loss 12 months or longer	0	2
Total Fair Value	1,355	1,639
Total Unrealized Loss	2	3
GNMA \| Residential Mortgage-backed Securities
Fair Value
Fair Value Less than 12 months	0	0
Unrealized Loss Less than 12 months	0	0
Fair Value 12 months or longer	389	520
Unrealized Loss 12 months or longer	25	26
Total Fair Value	389	520
Total Unrealized Loss	25	26
Government Sponsored Enterprises \| Residential Mortgage-backed Securities
Fair Value
Fair Value Less than 12 months	0	0
Unrealized Loss Less than 12 months	0	0
Fair Value 12 months or longer	385	500
Unrealized Loss 12 months or longer	19	18
Total Fair Value	385	500
Total Unrealized Loss	19	18
Government Sponsored Enterprises \| Commercial Mortgage-backed Securities
Fair Value
Fair Value Less than 12 months	0	0
Unrealized Loss Less than 12 months	0	0
Fair Value 12 months or longer	1,964	1,980
Unrealized Loss 12 months or longer	34	22
Total Fair Value	1,964	1,980
Total Unrealized Loss	$ 34	$ 22

Held to maturity securities unrealized loss position 12 months or longer accumulated loss.

No definition available.

Name:	fbiz_HeldToMaturitySecuritiesUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Held to maturity securities unrealized loss position accumulated loss.

No definition available.

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

Held to maturity securities unrealized loss position fair value.

No definition available.

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

Held to maturity securities unrealized loss position less than 12 months accumulated loss.

No definition available.

Name:	fbiz_HeldToMaturitySecuritiesUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Held to maturity securities unrealized loss position less than twelve months fair value.

No definition available.

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

Held to maturity securities unrealized loss position twelve months or longer fair value.

No definition available.

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

No definition available.

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

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=fbiz_GNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=fbiz_GovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: