Loans (Schedule of Defaults During Stated Period of Trouble Debt Restructurings) (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2016 USD ($) contract	Sep. 30, 2016 USD ($) contract
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	4	4
Recorded Investment at Default Date \| $	$ 454	$ 454
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Non-Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Land and Land Improvements [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Construction [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Non-Jumbo [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	3	3
Recorded Investment at Default Date \| $	$ 452	$ 452
Jumbo [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Home Equity [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Mortgage Warehouse Lines [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	0	0
Recorded Investment at Default Date \| $	$ 0	$ 0
Consumer Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Number of Defaults \| contract	1	1
Recorded Investment at Default Date \| $	$ 2	$ 2

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=66008989&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_OwnerOccupiedMember
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_ConstructionMember
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:

- Details

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

- Details

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

- Details

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

- Details

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