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Financial Instruments With Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Financial instruments with off-balance sheet risk (Textual) |  |
Commitment under letters of credit | $ 9.9 | $ 10.0
Commitments To Extend Credit [Member] |  |
Financial instruments with off-balance sheet risk (Textual) |  |
Amount of unfunded commitments to extend credit | 426.6 | 399.4
Unfunded commitments to extend credit at fixed rate | 105.0 | 130.5
Unfunded commitments to extend credit at variable rate | $ 321.6 | $ 269.0

X
- Definition Commitment under letters of credit. + References No definition available. + Details
Name: | sfst_CommitmentUnderLettersOfCredit
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | sfst_FinancialInstrumentsWithOffBalanceSheetRiskTextualAbstract
Namespace Prefix: | sfst_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Unfunded commitments to extend credit. + References No definition available. + Details
Name: | sfst_UnfundedCommitmentsToExtendCredit
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Unfunded commitments to extend credit at fixed rate. + References No definition available. + Details
Name: | sfst_UnfundedCommitmentsToExtendCreditAtFixedRate
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Unfunded commitments to extend credit at variable rate. + References No definition available. + Details
Name: | sfst_UnfundedCommitmentsToExtendCreditAtVariableRate
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |