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Derivative Financial Instruments (Narrative) (Details) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
Jan. 15, 2015
Derivative [Line Items]    
Number of forward-starting, pay-fixed/receive LIBOR interest rate swaps 3us-gaap_DerivativeNumberOfInstrumentsHeld  
Forward Contracts [Member] | Effective July 18, 2016 [Member]    
Derivative [Line Items]    
Notional Amount $ 40,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveJuly182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Notional Amount 40,000,000smmf_CashFlowHedgeDerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveJuly182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Derivative, swaption interest rate 2.98%us-gaap_DerivativeSwaptionInterestRate
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveJuly182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Derivative, term of contract 3 years  
Forward Contracts [Member] | Effective April 18, 2016 [Member]    
Derivative [Line Items]    
Notional Amount 30,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveApril182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Notional Amount 30,000,000smmf_CashFlowHedgeDerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveApril182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Derivative, swaption interest rate 2.89%us-gaap_DerivativeSwaptionInterestRate
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveApril182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Derivative, term of contract 4 years 6 months  
Forward Contracts [Member] | Effective October 18, 2016 [Member]    
Derivative [Line Items]    
Notional Amount 40,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveOctober182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Notional Amount 40,000,000smmf_CashFlowHedgeDerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveOctober182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Derivative, swaption interest rate 2.841%us-gaap_DerivativeSwaptionInterestRate
/ us-gaap_DerivativeByNatureAxis
= smmf_EffectiveOctober182016Member
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
 
Derivative, term of contract 3 years  
Fair Value Hedging [Member] | Commercial Loan [Member] | Interest Rate Swap [Member]    
Derivative [Line Items]    
Notional Amount $ 9,950,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
$ 9,950,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
Derivative, swaption interest rate   4.33%us-gaap_DerivativeSwaptionInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
Derivative, term of contract   10 years