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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: