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

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

- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of assets acquired at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of liabilities assumed at the acquisition date.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionAxis=smmf_HIghlandCountyBanksharesIncandSubsidiaryFirstandCitizensBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=smmf_FirstCenturyBanksharesIncMember
Namespace Prefix:
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Balance Type:
Period Type:

Acquisitions Acquisitions (Narrative)(Details) - USD ($) $ in Millions	Apr. 01, 2017	Mar. 31, 2017	Oct. 01, 2016	Sep. 30, 2016
HIghland County Bankshares Inc and Subsidiary First and Citizens Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired			100.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets				$ 123
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities				$ 107
First Century Bankshares, Inc [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		$ 406
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		$ 361