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Securities (Summary Of Amortized Cost, Unrealized Gains, Unrealized Losses And Estimated Fair Values) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost $ 276,631us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 289,614us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available for sale securities, Unrealized Gains 7,170us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 3,851us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Available for sale securities, Unrealized Losses 967us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax 4,685us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Available for sale securities, Estimated Fair Value 282,834smmf_AvailableForSaleSecuritiesFairValueDisclosure1 288,780smmf_AvailableForSaleSecuritiesFairValueDisclosure1
U.S. Government and agencies and corporations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 22,153us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember		 29,100us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Available for sale securities, Unrealized Gains 1,073us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember		 675us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Available for sale securities, Unrealized Losses 52us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember		 118us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Available for sale securities, Estimated Fair Value 23,174smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember		 29,657smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Government-sponsored agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 147,951us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember		 155,270us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
Available for sale securities, Unrealized Gains 2,599us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember		 2,019us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
Available for sale securities, Unrealized Losses 773us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember		 1,573us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
Available for sale securities, Estimated Fair Value 149,777smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember		 155,716smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
Nongovernment-sponsored entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 12,051us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember		 11,519us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
Available for sale securities, Unrealized Gains 142us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember		 321us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
Available for sale securities, Unrealized Losses 48us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember		 21us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
Available for sale securities, Estimated Fair Value 12,145smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember		 11,819smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
State and political subdivisions, General obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 1,975us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 9,317us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available for sale securities, Unrealized Gains 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available for sale securities, Unrealized Losses 33us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 475us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available for sale securities, Estimated Fair Value 1,944smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 8,842smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
State and policital subdivisions, Water and sewer revenues [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 1,976us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember		 3,229us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember
Available for sale securities, Unrealized Gains 14us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember
Available for sale securities, Unrealized Losses 7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember		 114us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember
Available for sale securities, Estimated Fair Value 1,983smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember		 3,115smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_WaterAndSewerRevenuesMember
State and policital subdivisions, Other revenues [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 4,696us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember		 4,051us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember
Available for sale securities, Unrealized Gains 73us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember		 4us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember
Available for sale securities, Unrealized Losses 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember		 142us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember
Available for sale securities, Estimated Fair Value 4,767smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember		 3,913smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_OtherRevenuesMember
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 3,776us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 3,973us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available for sale securities, Unrealized Gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 24us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available for sale securities, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 31us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available for sale securities, Estimated Fair Value 3,776smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 3,966smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total taxable debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 194,578us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember		 216,459us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember
Available for sale securities, Unrealized Gains 3,903us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember		 3,043us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember
Available for sale securities, Unrealized Losses 915us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember		 2,474us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember
Available for sale securities, Estimated Fair Value 197,566smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember		 217,028smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxableDebtSecuritiesMember
State and political subdivisions, General obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 49,515us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember		 41,156us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember
Available for sale securities, Unrealized Gains 2,338us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember		 675us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember
Available for sale securities, Unrealized Losses 12us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember		 1,154us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember
Available for sale securities, Estimated Fair Value 51,841smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember		 40,677smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsGeneralObligationsMember
State and political subdivisions, Water and sewer revenues [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 11,258us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember		 8,996us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember
Available for sale securities, Unrealized Gains 244us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember		 15us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember
Available for sale securities, Unrealized Losses 3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember		 306us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember
Available for sale securities, Estimated Fair Value 11,499smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember		 8,705smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsWaterAndSewerRevenuesMember
State and political subdivisions, Lease revenues [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 4,617us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember		 7,956us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember
Available for sale securities, Unrealized Gains 75us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember
Available for sale securities, Unrealized Losses 10us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember		 391us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember
Available for sale securities, Estimated Fair Value 4,682smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember		 7,565smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLeaseRevenuesMember
State and political subdivisions, Lottery/casino revenues [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 3,811us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember		 4,443us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember
Available for sale securities, Unrealized Gains 206us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember		 63us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember
Available for sale securities, Unrealized Losses 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember		 169us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember
Available for sale securities, Estimated Fair Value 4,008smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember		 4,337smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsLotteryCasinoRevenuesMember
State and political subdivisions, Other revenues [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 12,845us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember		 10,527us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember
Available for sale securities, Unrealized Gains 404us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember		 55us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember
Available for sale securities, Unrealized Losses 18us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember		 191us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember
Available for sale securities, Estimated Fair Value 13,231smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember		 10,391smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_StateAndPoliticalSubdivisionsOtherRevenuesMember
Total tax-exempt debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 82,046us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember		 73,078us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember
Available for sale securities, Unrealized Gains 3,267us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember		 808us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember
Available for sale securities, Unrealized Losses 52us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember		 2,211us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember
Available for sale securities, Estimated Fair Value 85,261smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember		 71,675smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesMember
Equity securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 7us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 77us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Available for sale securities, Unrealized Gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Available for sale securities, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Available for sale securities, Estimated Fair Value $ 7smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 $ 77smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember