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Fair Value Accounting (Schedule of Assets And Liabilities Measured at Fair Value on Nonrecurring Basis) (Details) - Fair Value, Measurements, Nonrecurring [Member] - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
Assets |  |
Impaired loans | $ 10,547 | $ 11,253
Other real estate owned |  | 1,169
Total assets measured at fair value on a nonrecurring basis | 10,547 | 12,422
Fair Value, Inputs, Level 1 [Member] |  |
Assets |  |
Impaired loans |  |
Other real estate owned |  |
Total assets measured at fair value on a nonrecurring basis |  |
Fair Value, Inputs, Level 2 [Member] |  |
Assets |  |
Impaired loans | 7,833 | 8,144
Other real estate owned |  | 1,169
Total assets measured at fair value on a nonrecurring basis | 7,833 | 9,313
Fair Value, Inputs, Level 3 [Member] |  |
Assets |  |
Impaired loans | 2,714 | 3,109
Other real estate owned |  |
Total assets measured at fair value on a nonrecurring basis | $ 2,714 | $ 3,109

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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
Name: | sfst_AssetsFairValueDisclosuresNonRecurring
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Impaired loans, Fair value. + References No definition available. + Details
Name: | sfst_ImpairedLoansFairValue
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Other real estate owned. + References No definition available. + Details
Name: | sfst_OtherRealEstateOwned
Namespace Prefix: | sfst_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |