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Commitments and Contingencies - SBA Recourse Reserve (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Commitments and Contingencies Disclosure [Abstract]
Beginning balance	$ 507	$ 906	$ 645	$ 955
SBA recourse (benefit) expense	(5)	466	(64)	583
Charge-offs, net	(0)	(40)	(79)	(206)
Ending balance	$ 502	$ 1,332	$ 502	$ 1,332

X

- Definition

SBA Recourse Provision (Benefit)

+ References

No definition available.

+ Details

Name:	fbiz_SBARecourseProvisionBenefit
Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

SBA Recourse Reserve

+ References

No definition available.

+ Details

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

X

- Definition

SBA Loan Charge Offs, Net

+ References

No definition available.

+ Details

Name:	fbiz_SbaLoanChargeOffsNet
Namespace Prefix:	fbiz_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

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