Represents the basis spread adjustment on a debt instrument.

No definition available.

Name:	sfdl_DebtInstrumentBasisSpreadAdjustment
Namespace Prefix:	sfdl_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-4

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
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+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
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-URI https://asc.fasb.org/1943274/2147482949/835-30-55-8

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+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(a)(1))
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRepurchaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=sfdl_JuniorSubordinatedDebenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=sfdl_LiborMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_SecuredOvernightFinancingRateSofrMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sfdl_TenYearNotesDue2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sfdl_FifteenYearNotesDue2034Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Note 11 - Subordinated Debentures (Details Textual) $ in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Nov. 30, 2019 USD ($)	Sep. 30, 2006	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022 USD ($)
Junior Subordinated Debentures [Member]
Debt Instrument, Interest Rate, Effective Percentage			7.29%	7.35%
Junior Subordinated Debentures [Member] \| LIBOR [Member]
Debt Instrument, Basis Spread on Variable Rate		1.70%		1.70%
Junior Subordinated Debentures [Member] \| Secured Overnight Financing Rate (SOFR) [Member]
Debt Instrument, Basis Spread on Variable Rate			1.70%
Debt Instrument, Basis Spread Adjustment			0.26161
Ten Year Notes Due 2029 [Member] \| Subordinated Debt [Member]
Debt Instrument, Face Amount	$ 17.5
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Debt Instrument, Repurchase Amount					$ 1.0
Long-Term Debt, Gross					16.5
Ten Year Notes Due 2029 [Member] \| LIBOR [Member] \| Subordinated Debt [Member]
Debt Instrument, Basis Spread on Variable Rate	3.69%
Fifteen Year Notes Due 2034 [Member] \| Subordinated Debt [Member]
Debt Instrument, Face Amount	$ 12.5
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Debt Instrument, Repurchase Amount					2.5
Long-Term Debt, Gross					$ 10.0
Fifteen Year Notes Due 2034 [Member] \| LIBOR [Member] \| Subordinated Debt [Member]
Debt Instrument, Basis Spread on Variable Rate	3.57%