XML 90 R83.htm IDEA: XBRL DOCUMENT v3.25.3

Fair Values of Assets and Liabilities - Summary of Significant Unobservable Inputs used in Level 3 Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Inputs, Level 3 [Member] - Fair Value, Measurements, Recurring [Member]
$ in Thousands

9 Months Ended

12 Months Ended

Sep. 30, 2025

USD ($)

Dec. 31, 2024

USD ($)

Sep. 30, 2024

USD ($)

Dec. 31, 2023

USD ($)

Mortgage servicing rights [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights

^[1]

$ 267

$ 305

$ 308

$ 339

Prepayment Rate [Member] | Mortgage servicing rights [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Servicing Asset, Valuation Technique [Extensible List]

us-gaap:ValuationTechniqueDiscountedCashFlowMember

us-gaap:ValuationTechniqueDiscountedCashFlowMember

Mortgage Servicing Rights

$ 267

$ 305

Prepayment Rate [Member] | Minimum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.0467

0.0454

Prepayment Rate [Member] | Maximum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.254

0.1871

Prepayment Rate [Member] | Weighted Average [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

^[2]

0.0903

0.0651

Fair value valuation techniques

Prepayment Rate

Prepayment Rate

Discount Rate [Member] | Minimum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.095

0.10

Discount Rate [Member] | Maximum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.095

0.10

Discount Rate [Member] | Weighted Average [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

^[2]

0.0950

0.1000

Fair value valuation techniques

Discount Rate

Discount Rate

Delinquency Rate [Member] | Minimum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.0211

0.0217

Delinquency Rate [Member] | Maximum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.0252

0.0264

Delinquency Rate [Member] | Weighted Average [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

^[2]

0.0218

0.0225

Fair value valuation techniques

Delinquency Rate

Delinquency Rate

Default Rate [Member] | Minimum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.0014

0.0014

Default Rate [Member] | Maximum [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

0.0024

0.0024

Default Rate [Member] | Weighted Average [Member]

Fair Value Measurement Inputs and Valuation Techniques [Line Items]

Mortgage Servicing Rights, Measuring Inputs

^[2]

0.0016

0.0016

Fair value valuation techniques

Default Rate

Default Rate

[1]	Realized and unrealized gains and losses related to mortgage servicing rights are reported as a component of loan servicing fee income in the Company’s consolidated statements of income (loss).
[2]	Unobservable inputs for mortgage servicing rights were weighted by loan amount.

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 43
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-43

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 103
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-103

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 101
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-101

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

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

X

- Definition

Value of input used to measure servicing asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_ServicingAssetMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Indicates valuation technique for measuring servicing asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_ServicingAssetValuationTechniqueExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Description of valuation technique or method used to estimate fair value of servicing assets and servicing liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-SubTopic 50
-Topic 860
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-2

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesFairValueValuationTechniques
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByAssetClassAxis=fsea_MortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputPrepaymentRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=fsea_MeasurementInputDelinquencyRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDefaultRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: