Loans Loans (Schedule of Defaults During Stated Period Of Trouble Debt Restructurings) (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($) contract	Mar. 31, 2018 USD ($) contract
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| contract	10	3
Financing Receivable, Modifications, Subsequent Default, Recorded Investment \| $	$ 1,043	$ 843
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| contract	2	0
Financing Receivable, Modifications, Subsequent Default, Recorded Investment \| $	$ 157	$ 0
Non-Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| contract	1	1
Financing Receivable, Modifications, Subsequent Default, Recorded Investment \| $	$ 126	$ 341
Land and Land Improvements [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| contract	0	1
Financing Receivable, Modifications, Subsequent Default, Recorded Investment \| $	$ 0	$ 438
Non-Jumbo [Member]
Financing Receivable, Impaired [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts \| contract	7	1
Financing Receivable, Modifications, Subsequent Default, Recorded Investment \| $	$ 760	$ 64

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=smmf_NonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_LandAndLandImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=smmf_NonJumboMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: