Commitments And Contingencies Commitments and Contingencies (Narrative) (Details) - USD ($)	3 Months Ended
Mar. 31, 2020	Jan. 01, 2020
Commitments and Contingencies Disclosure [Abstract]
Impact to ACL on off-balance sheet credit exposures upon adoption of ASU 2016-13		$ 2,430,000
Off Balance Sheet Credit Loss Liability Credit Loss Expense Reversal	$ 551,000
Off Balance Sheet Credit Loss Liability	$ 2,980,000

Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

No definition available.

Name:	smmf_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal
Namespace Prefix:	smmf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration