Loans Loans (Schedule of Restructured Trouble Debt Restructuring By Class) (Details) $ in Thousands	3 Months Ended
Mar. 31, 2016 USD ($) contract	Mar. 31, 2015 contract
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	1	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 250
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 250
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Owner Occupied [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Non-Owner Occupied [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Land and Land Improvements [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Construction [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Non-Jumbo [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	1
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 250
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 250
Jumbo [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0
Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts \| contract	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 0
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0

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_FinancingReceivableModificationsLineItems
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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
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=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: