Derivative Financial Instruments Derivative Financial Instruments (Narrative) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Number of instruments held	3
Cash flow hedges, notional amount	$ 110,000,000	$ 70,000,000
Forward Contracts [Member] \| Effective July 18, 2016 [Member]
Derivative [Line Items]
Derivative, notional amount	40,000,000
Cash flow hedges, notional amount	40,000,000
Swaption interest rate	2.98%
Term of contract	3 years
Forward Contracts [Member] \| Effective April 18, 2016 [Member]
Derivative [Line Items]
Derivative, notional amount	30,000,000
Cash flow hedges, notional amount	30,000,000
Swaption interest rate	2.89%
Term of contract	4 years 6 months
Forward Contracts [Member] \| Effective October 18, 2016 [Member]
Derivative [Line Items]
Derivative, notional amount	40,000,000
Cash flow hedges, notional amount	$ 40,000,000
Swaption interest rate	2.841%
Term of contract	3 years

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

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Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Cash Flow Hedge Derivative Liability Notional Amount.

+ References

No definition available.

+ Details

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Namespace Prefix:	smmf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959

+ Details

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- Definition

Fixed interest rate that will be received or paid upon exercise of the interest rate swaption contract.

+ References

No definition available.

+ Details

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

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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