Significant Events Narrative (Details) $ in Millions	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Significant Events [Abstract]
Number of Short-Term Loan Modifications	7
Gross Loans and Leases Outstanding, Short-Term Modification	$ 13.0	$ 27.0
Gross Loans and Leases Outstanding, Short-Term Modification, Percent 6 Month Deferral Period	0.70%	1.40%

Loan deferrals of six months as a percent of the total gross loans and leases deferred due to COVID-19

No definition available.

Name:	fbiz_GrossLoansandLeasesOutstandingShortTermModificationPercent6MonthDeferralPeriod
Namespace Prefix:	fbiz_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant