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Outstanding Commitments and Contingent Liabilities (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Credit Commitments and Contingencies    
Liabilities for potential obligations under guarantees 0buse_LossContingencyLiabilityForPotentialObligationsUnderGuarantees $ 0buse_LossContingencyLiabilityForPotentialObligationsUnderGuarantees
Maximum    
Credit Commitments and Contingencies    
Term for guarantee 1 year  
Commitments to extend credit    
Credit Commitments and Contingencies    
Financial instruments whose contract amounts represent credit risk 592,716us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember		 561,439us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember
Standby letters of credit    
Credit Commitments and Contingencies    
Financial instruments whose contract amounts represent credit risk 21,046us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember		 $ 20,466us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember