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Fair Value Measurements (Assets And Liabilities Recorded At Fair Value On A Recurring Basis) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value $ 282,135us-gaap_AvailableForSaleSecurities $ 282,834us-gaap_AvailableForSaleSecurities [1] $ 281,865us-gaap_AvailableForSaleSecurities
U.S. Government sponsored agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 22,837us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
23,174us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
 
Government Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 156,815us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
149,777us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
 
Nongovernment Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 11,068us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
12,145us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
 
U S States And Political Subdivisions General Obligations [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 8,470us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
8,694us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Corporate Debt Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 6,665us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
3,776us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Equity Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 7us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
7us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Tax Exempt Debt Securities Us States And Political Subdivisions [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 76,273us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
85,261us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
 
Interest rate swap [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate swap 4,292smmf_InterestRateSwap
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
2,911smmf_InterestRateSwap
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
 
Fair Value Inputs Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
 
Fair Value Inputs Level 1 [Member] | U.S. Government sponsored agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
 
Fair Value Inputs Level 1 [Member] | Government Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
 
Fair Value Inputs Level 1 [Member] | Nongovernment Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
 
Fair Value Inputs Level 1 [Member] | U S States And Political Subdivisions General Obligations [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Fair Value Inputs Level 1 [Member] | Corporate Debt Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Fair Value Inputs Level 1 [Member] | Equity Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Fair Value Inputs Level 1 [Member] | Tax Exempt Debt Securities Us States And Political Subdivisions [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
 
Fair Value Inputs Level 1 [Member] | Interest rate swap [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate swap 0smmf_InterestRateSwap
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
0smmf_InterestRateSwap
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
 
Fair Value Inputs Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 278,346us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
279,058us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
 
Fair Value Inputs Level 2 [Member] | U.S. Government sponsored agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 22,837us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
23,174us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
 
Fair Value Inputs Level 2 [Member] | Government Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 156,815us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
149,777us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
 
Fair Value Inputs Level 2 [Member] | Nongovernment Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 11,068us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
12,145us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
 
Fair Value Inputs Level 2 [Member] | U S States And Political Subdivisions General Obligations [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 8,470us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
8,694us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Fair Value Inputs Level 2 [Member] | Corporate Debt Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 2,876us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Fair Value Inputs Level 2 [Member] | Equity Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 7us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
7us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Fair Value Inputs Level 2 [Member] | Tax Exempt Debt Securities Us States And Political Subdivisions [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 76,273us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
85,261us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
 
Fair Value Inputs Level 2 [Member] | Interest rate swap [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate swap 4,292smmf_InterestRateSwap
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
2,911smmf_InterestRateSwap
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
 
Fair Value Inputs Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 3,789us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
3,776us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
 
Fair Value Inputs Level 3 [Member] | U.S. Government sponsored agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
 
Fair Value Inputs Level 3 [Member] | Government Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_GovernmentSponsoredAgenciesMember
 
Fair Value Inputs Level 3 [Member] | Nongovernment Sponsored Agencies [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_NongovernmentSponsoredAgenciesMember
 
Fair Value Inputs Level 3 [Member] | U S States And Political Subdivisions General Obligations [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Fair Value Inputs Level 3 [Member] | Corporate Debt Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 3,789us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
3,776us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Fair Value Inputs Level 3 [Member] | Equity Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Fair Value Inputs Level 3 [Member] | Tax Exempt Debt Securities Us States And Political Subdivisions [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available for sale securities, fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= smmf_TaxExemptDebtSecuritiesUsStatesAndPoliticalSubdivisionsMember
 
Fair Value Inputs Level 3 [Member] | Interest rate swap [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate swap $ 0smmf_InterestRateSwap
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
$ 0smmf_InterestRateSwap
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestRateSwapMember
 
Minimum [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Discount to sell collateral 7.00%smmf_CollateralizedFinancingsDiscounttoSellCollateral
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Maximum [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Discount to sell collateral 10.00%smmf_CollateralizedFinancingsDiscounttoSellCollateral
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
[1] December 31, 2014 financial information has been extracted from audited consolidated financial statements