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Securities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Securities pledged to secure public deposits and other purposes 128.1us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue $ 120.0us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
Securities held with an unrealized loss position 57smmf_NumberSecuritiesHeldWithUnrealizedLossPosition  
Residential Mortgage Backed Securities [Member] | Minimum [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Contractual maturity 3 years  
Estimated average life to maturity 1 year  
Residential Mortgage Backed Securities [Member] | Maximum [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Contractual maturity 50 years  
Estimated average life to maturity 33 years