Significant Events CARES Act (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Significant Events [Abstract]
SBA Paycheck Protection Program Loans Outstanding, Gross Amount	$ 332,300	$ 332,300
SBA Paycheck Protection Program, Loan & Leases Receiveable, Deferred Income	6,900	6,900
SBA Paycheck Protection Program Interest Income, Loans	$ 1,100	$ 2,000

SBA Paycheck Protection Program, Loan & Leases Receiveable, Deferred Income

No definition available.

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