Property Held For Sale Property Held for Sale (Narrative) (Details)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment Assets Held-for-sale Disclosure [Abstract]
Foreclosed Property Consumer Residential Real Estate	$ 1,100,000

Foreclosed Property Consumer Residential Real Estate

No definition available.

Name:	smmf_ForeclosedPropertyConsumerResidentialRealEstate
Namespace Prefix:	smmf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant