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Securities (Summary of Volume of State and Political Subdivision Securities Held in Portfolio) (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
California [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 15,060us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
 
Available for sale securities, Unrealized Gains 597us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
 
Available for sale securities, Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
 
Available for sale securities, Estimated Fair Value 15,657smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_CaliforniaMember
 
West Virginia [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 14,181us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
 
Available for sale securities, Unrealized Gains 355us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
 
Available for sale securities, Unrealized Losses 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
 
Available for sale securities, Estimated Fair Value 14,527smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_WestVirginiaMember
 
Illinois [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 11,275us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
 
Available for sale securities, Unrealized Gains 360us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
 
Available for sale securities, Unrealized Losses 19us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
 
Available for sale securities, Estimated Fair Value 11,616smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_IllinoisMember
 
Ohio [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 6,772us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
 
Available for sale securities, Unrealized Gains 69us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
 
Available for sale securities, Unrealized Losses 42us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
 
Available for sale securities, Estimated Fair Value 6,799smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_OhioMember
 
Texas [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 5,699us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_TexasMember
 
Available for sale securities, Unrealized Gains 410us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_TexasMember
 
Available for sale securities, Unrealized Losses 3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_TexasMember
 
Available for sale securities, Estimated Fair Value $ 6,106smmf_AvailableForSaleSecuritiesFairValueDisclosure1
/ us-gaap_StatementBusinessSegmentsAxis
= smmf_TexasMember