Securities Available-for-Sale - Summary of gross unrealized losses and fair value for those investments with unrealized losses (Detail) $ in Thousands	Jun. 30, 2025 USD ($) Number	Dec. 31, 2024 USD ($) Number
Less than 12 Months, Number of Securities \| Number	29	35
Less than 12 Months, Fair Value	$ 38,130	$ 39,378
Less than 12 Months, Unrealized Losses	$ (437)	$ (588)
More than 12 Months, Number of Securities \| Number	82	81
More than 12 Months, Fair Value	$ 55,490	$ 57,092
More than 12 Months, Unrealized Losses	(10,273)	(9,688)
Total, Fair Value	93,620	96,470
Total, Unrealized Losses	$ (10,710)	$ (10,276)
U.S. Government-sponsored enterprises obligations [Member]
More than 12 Months, Number of Securities \| Number	3	3
More than 12 Months, Fair Value	$ 1,438	$ 1,387
More than 12 Months, Unrealized Losses	(190)	(255)
Total, Fair Value	1,438	1,387
Total, Unrealized Losses	$ (190)	$ (255)
U.S. Government agency small business administration pools guaranteed by the SBA [Member]
Less than 12 Months, Number of Securities \| Number	7	5
Less than 12 Months, Fair Value	$ 5,310	$ 4,245
Less than 12 Months, Unrealized Losses	$ (42)	$ (28)
More than 12 Months, Number of Securities \| Number	8	8
More than 12 Months, Fair Value	$ 5,962	$ 6,224
More than 12 Months, Unrealized Losses	(717)	(874)
Total, Fair Value	11,272	10,469
Total, Unrealized Losses	$ (759)	$ (902)
Collateralized mortgage obligations issued by the FHLMC, FNMA and GNMA [Member]
Less than 12 Months, Number of Securities \| Number	8	7
Less than 12 Months, Fair Value	$ 17,779	$ 11,913
Less than 12 Months, Unrealized Losses	$ (192)	$ (126)
More than 12 Months, Number of Securities \| Number	4	4
More than 12 Months, Fair Value	$ 2,244	$ 2,256
More than 12 Months, Unrealized Losses	(386)	(470)
Total, Fair Value	20,023	14,169
Total, Unrealized Losses	$ (578)	$ (596)
Residential mortgage backed securities [Member]
Less than 12 Months, Number of Securities \| Number	9	20
Less than 12 Months, Fair Value	$ 12,987	$ 22,395
Less than 12 Months, Unrealized Losses	$ (156)	$ (422)
More than 12 Months, Number of Securities \| Number	25	24
More than 12 Months, Fair Value	$ 14,714	$ 14,485
More than 12 Months, Unrealized Losses	(3,455)	(3,960)
Total, Fair Value	27,701	36,880
Total, Unrealized Losses	$ (3,611)	$ (4,382)
Municipal bonds [Member]
Less than 12 Months, Number of Securities \| Number	5	3
Less than 12 Months, Fair Value	$ 2,054	$ 825
Less than 12 Months, Unrealized Losses	$ (47)	$ (12)
More than 12 Months, Number of Securities \| Number	36	36
More than 12 Months, Fair Value	$ 25,686	$ 27,369
More than 12 Months, Unrealized Losses	(4,970)	(3,490)
Total, Fair Value	27,740	28,194
Total, Unrealized Losses	$ (5,017)	$ (3,502)
Corporate debt [Member]
More than 12 Months, Number of Securities \| Number	1	1
More than 12 Months, Fair Value	$ 487	$ 484
More than 12 Months, Unrealized Losses	(13)	(16)
Total, Fair Value	487	484
Total, Unrealized Losses	$ (13)	$ (16)
Corporate subordinated debt [Member]
More than 12 Months, Number of Securities \| Number	5	5
More than 12 Months, Fair Value	$ 4,959	$ 4,887
More than 12 Months, Unrealized Losses	(542)	(623)
Total, Fair Value	4,959	4,887
Total, Unrealized Losses	$ (542)	$ (623)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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