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Securities (Summary of Volume of State and Political Subdivision Securities Held in Portfolio) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]      
Total Amortized Cost $ 275,204us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 276,631us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 279,511us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available for sale securities, Unrealized Gains 7,732us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 7,170us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 4,820us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Available for sale securities, Unrealized Losses 801us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax 967us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax 2,466us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Available for sale securities, Estimated Fair Value 282,135smmf_AvailableForSaleSecuritiesFairValueDisclosure1 282,834smmf_AvailableForSaleSecuritiesFairValueDisclosure1 281,865smmf_AvailableForSaleSecuritiesFairValueDisclosure1
West Virginia [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Total Amortized Cost 13,083us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
   
Available for sale securities, Unrealized Gains 314us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
   
Available for sale securities, Unrealized Losses 1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
   
Available for sale securities, Estimated Fair Value 13,396smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
   
Illinois [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Total Amortized Cost 12,503us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
   
Available for sale securities, Unrealized Gains 378us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
   
Available for sale securities, Unrealized Losses 26us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
   
Available for sale securities, Estimated Fair Value 12,855smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
   
California [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Total Amortized Cost 10,999us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
   
Available for sale securities, Unrealized Gains 527us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
   
Available for sale securities, Unrealized Losses 25us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
   
Available for sale securities, Estimated Fair Value 11,501smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
   
Ohio [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Total Amortized Cost 7,621us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
   
Available for sale securities, Unrealized Gains 39us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
   
Available for sale securities, Unrealized Losses 26us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
   
Available for sale securities, Estimated Fair Value 7,634smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
   
Washington [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Total Amortized Cost 5,845us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= stpr_WA
   
Available for sale securities, Unrealized Gains 283us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= stpr_WA
   
Available for sale securities, Unrealized Losses 40us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= stpr_WA
   
Available for sale securities, Estimated Fair Value $ 6,088smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= stpr_WA