Troubled Debt Restructurings (Details Textual) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) Investments	Dec. 31, 2018 USD ($) Investments
Troubled Debt Restructurings (Textual)
Total number of loans classified under troubled debt restructurings (TDRs)	19	26
Total sum of loans classified as troubled debt restructurings (TDRs) \| $	$ 9.3	$ 9.3
Number of months previous loan payment defaulted	12 months	12 months
Commercial [Member]
Troubled Debt Restructurings (Textual)
Total number of loans added under troubled debt restructurings (TDRs)	1
Total sum of loans added as troubled debt restructurings (TDRs) \| $	$ 1.8
Total number of loans removed under TDR	9
Total number of nonaccrual loans restored	5

Aggregate number of loans added under troubled debt restructurings.

No definition available.

Name:	sfst_AggregateNumberOfLoansAddedUnderTroubledDebtRestructurings
Namespace Prefix:	sfst_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Aggregate number of loans classified under troubled debt restructurings.

No definition available.

Name:	sfst_AggregateNumberOfLoansClassifiedUnderTroubledDebtRestructurings
Namespace Prefix:	sfst_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Aggregate number of loans removed under troubled debt restructurings.

No definition available.

Name:	sfst_AggregateNumberOfLoansRemovedUnderTroubledDebtRestructurings
Namespace Prefix:	sfst_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Aggregate sum of loans added as troubled debt restructurings.

No definition available.

Name:	sfst_AggregateSumOfLoansAddedAsTroubledDebtRestructurings
Namespace Prefix:	sfst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Aggregate sum of loans classified as troubled debt restructurings.

No definition available.

Name:	sfst_AggregateSumOfLoansClassifiedAsTroubledDebtRestructurings
Namespace Prefix:	sfst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Number of month payment defaulted.

No definition available.

Name:	sfst_NumberOfMonthPaymentDefaulted
Namespace Prefix:	sfst_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Restored nonaccrual loans.

No definition available.

Name:	sfst_RestoredNonaccrualLoans
Namespace Prefix:	sfst_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

It covers narrative parts of troubled debt restructurings note.

No definition available.

Name:	sfst_TroubledDebtRestructuringsTextualAbstract
Namespace Prefix:	sfst_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: