Securities (Summary of Volume of State and Political Subdivision Securities Held in Portfolio) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$ 324,908	$ 267,105
Available for sale securities, Unrealized Gains	5,329	3,005
Available for sale securities, Unrealized Losses	1,514	3,568
Available for sale securities, Estimated Fair Value	328,723	$ 266,542
TEXAS
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	20,055
Available for sale securities, Unrealized Gains	611
Available for sale securities, Unrealized Losses	48
Available for sale securities, Estimated Fair Value	20,618
MICHIGAN [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	19,362
Available for sale securities, Unrealized Gains	292
Available for sale securities, Unrealized Losses	94
Available for sale securities, Estimated Fair Value	19,560
CALIFORNIA
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	12,265
Available for sale securities, Unrealized Gains	411
Available for sale securities, Unrealized Losses	21
Available for sale securities, Estimated Fair Value	12,655
ILLINOIS
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	11,125
Available for sale securities, Unrealized Gains	214
Available for sale securities, Unrealized Losses	12
Available for sale securities, Estimated Fair Value	11,327
NEW YORK [Domain]
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	10,732
Available for sale securities, Unrealized Gains	298
Available for sale securities, Unrealized Losses	18
Available for sale securities, Estimated Fair Value	$ 11,012

Available For Sale Securities Fair Value Disclosure 1.

No definition available.

Name:	smmf_AvailableForSaleSecuritiesFairValueDisclosure1
Namespace Prefix:	smmf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=stpr_TX
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=smmf_MICHIGANMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=stpr_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=stpr_IL
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=smmf_NEWYORKDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: