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Financial Instruments With Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Financial instruments with off-balance sheet risk (Textual) |  |
Commitment under letters of credit | $ 8.7 | $ 9.9
Commitments To Extend Credit [Member] |  |
Financial instruments with off-balance sheet risk (Textual) |  |
Amount of unfunded commitments to extend credit | 480.1 | 426.6
Unfunded commitments to extend credit at fixed rate | 114.6 | 105.0
Unfunded commitments to extend credit at variable rate | $ 365.5 | $ 321.6

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: |