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Fair Value Measurements (Narrative) (Details)
Dec. 31, 2014
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