Revenue Recognition Revenue from Contracts with Customers (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Revenue from Contracts with Customers [Line Items]
GainLossOnSalesOfForeclosedProperties	$ 64	$ 156
Revenue within other ASC Scopes [Member]
Revenue from Contracts with Customers [Line Items]
GainLossOnSalesOfForeclosedProperties	$ 64

[Line Items] for Revenue from Contracts with Customers [Table]

No definition available.

Name:	smmf_RevenuefromContractswithCustomersLineItems
Namespace Prefix:	smmf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration