Revenue Recognition Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Revenue from Contracts with Customers [Line Items]
Service fees related to deposit accounts	$ 1,091	$ 683
Fees and Commissions, Credit and Debit Cards	749	534
Fees and Commissions, Fiduciary and Trust Activities	667	100
Insurance commissions	1,113	968
Noninterest Income, Other Operating Income	249	102
Non-interest income within the scope of other ASC topics	1,203
Noninterest Income	4,876	$ 2,579
Revenue in Scope ASC 606 [Member]
Revenue from Contracts with Customers [Line Items]
Service fees related to deposit accounts	1,091
Fees and Commissions, Credit and Debit Cards	749
Fees and Commissions, Fiduciary and Trust Activities	667
Insurance commissions	1,113
Noninterest Income, Other Operating Income	53
Total Revenue in ASC 606	$ 3,673

Non-interest income within the scope of other ASC topics

No definition available.

Name:	smmf_NoninterestincomewithinthescopeofotherASCtopics
Namespace Prefix:	smmf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

[Line Items] for Revenue from Contracts with Customers [Table]

+ References

No definition available.

+ Details

Name:	smmf_RevenuefromContractswithCustomersLineItems
Namespace Prefix:	smmf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Total Revenue in ASC 606

+ References

No definition available.

+ Details

Name:	smmf_TotalRevenueinASC606
Namespace Prefix:	smmf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of income earned from credit card services (including late, over limit, and annual fees) and debit card services (including interchange fees, annual fees and other fees). Excludes interest income earned on credit card balances and transactions.

+ References

No definition available.

+ Details

Name:	us-gaap_FeesAndCommissionsCreditAndDebitCards
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Includes amounts charged depositors for: (1) maintenance of their accounts (maintenance charges); (2) failure to maintain specified minimum balances on account; (3) exceeding the number of checks or transactions allowed to be processed in a given period; (4) checks drawn on no minimum balance deposit accounts; (5) withdrawals from nontransaction deposit accounts; (6) closing savings accounts before a specified minimum period of time has elapsed; (7) accounts which have remained inactive for extended periods of time or which have become dormant; (8) use of automated teller machines or remote service units; (9) checks drawn against insufficient funds that the bank assesses regardless of whether it decides to pay, return or hold the check; (10) issuing stop payment orders; (11) certifying checks; and (12) accumulating or disbursing funds deposited in IRAs or Keogh Plan accounts when not handled by the bank's trust department. This item does not include penalties assessed on the early withdrawal of time deposits.

+ References

+ Details

Name:	us-gaap_FeesAndCommissionsDepositorAccounts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income from services rendered by trust departments or by any consolidated subsidiaries acting in a fiduciary capacity, including fees earned for (1) acting as an agent for fiduciary placement and loans and (2) custody or investment advisory services when such fees are not separately billed to the customer. This item excludes commissions and fees received for the accumulation or disbursement of funds deposited to Individual Retirement or Keogh plan accounts when such are not handled by the reporting entity's trust department.

+ References

+ Details

Name:	us-gaap_FeesAndCommissionsFiduciaryAndTrustActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.4)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(c),(d))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_InsuranceCommissionsAndFees
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The total amount of noninterest income which may be derived from: (1) fees and commissions; (2) premiums earned; (3) insurance policy charges; (4) the sale or disposal of assets; and (5) other sources not otherwise specified.

+ References

+ Details

Name:	us-gaap_NoninterestIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue earned, classified as other, excluding interest income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-03(1)(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20187-122688

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(4))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

+ Details

Name:	us-gaap_NoninterestIncomeOtherOperatingIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	smmf_RevenuefromContractswithCustomersAxis=smmf_RevenueinScopeASC606Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: