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Securities (Contractual Maturity) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Available-for-Sale, Amortized Cost    
Due in one year or less $ 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis  
Due in one year through five years 9,341us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis  
Due in five through ten years 75,842us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis  
Due in over ten years 55,941us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis  
Amortized cost 141,124us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis 143,636us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Available-for-Sale, Estimated Fair Value    
Due in one year or less 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue  
Due in one year through five years 9,375us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue  
Due in five through ten years 76,931us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue  
Due in over ten years 56,645us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue  
Estimated fair value 142,951us-gaap_AvailableForSaleSecurities 144,698us-gaap_AvailableForSaleSecurities
Held-to-Maturity, Amortized Cost    
Due in one year or less 0fbiz_HeldtomaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCost  
Due in one year through five years 3,261fbiz_HeldtomaturitySecuritiesDebtMaturitiesRollingYearsTwoThroughFiveAmortizedCost  
Due in five through ten years 13,330fbiz_HeldtomaturitySecuritiesDebtMaturitiesRollingYearsSixThroughTenAmortizedCost  
Due in over ten years 24,008fbiz_HeldtomaturitySecuritiesDebtMaturitiesRollingAfterTenYearsAmortizedCost  
Amortized cost 40,599us-gaap_HeldToMaturitySecurities 41,563us-gaap_HeldToMaturitySecurities
Held-to-Maturity, Estimated Fair Value    
Due in one year or less 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue  
Due in one year through five years 3,272us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue  
Due in five through ten years 13,532us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue  
Due in over ten years 24,343us-gaap_HeldToMaturitySecuritiesDebtMaturitiesRollingAfterTenYearsFairValue  
Estimated Fair Value $ 41,147us-gaap_HeldToMaturitySecuritiesFairValue $ 41,694us-gaap_HeldToMaturitySecuritiesFairValue