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Loans (Narrative) (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Loans and Leases Receivable Disclosure [Abstract]    
Residential real estate loan threshold for jumbo loans $ 600,000smmf_ResidentialRealEstateLoanThresholdForJumboLoanQualification  
Threshold period past due for nonaccrual status of financing receivable 90 days  
Threshold period past due for write-off of credit card loans 180 days  
Threshold period special event notification for writeoff of financing receivable 60 days  
Threshold period past due for writeoff of residential mortgage loans 180 days  
Threshold period for writeoff of collateralized consumer loans 120 days  
Risk rate loans, aggregate balance threshold 2,000,000.0smmf_RiskRateLoansAggregateBalanceThreshold  
Risk rate loans, loan balance threshold 500,000smmf_RiskRateLoansLoanBalanceThreshold  
Troubled debt restructurings included in impaired loans 32,900,000us-gaap_FinancingReceivableModificationsRecordedInvestment 34,700,000us-gaap_FinancingReceivableModificationsRecordedInvestment
Current Troubled debt restructurings included in impaired loans 30,200,000smmf_CurrentFinancingReceivableModificationsRecordedInvestment 32,200,000smmf_CurrentFinancingReceivableModificationsRecordedInvestment
Troubled debt restructuring, aggregate exposure $ 2,000,000smmf_NonHomogenousCommercialLoanRelationshipAnnualLoanReviewThreshold