Revenue Recognition Revenue from Contracts with Customers (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018
Revenue within other ASC Scopes [Member]
Revenue from Contracts with Customers [Line Items]
GainLossOnSalesOfForeclosedProperties	$ (18)	$ (12)

[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