Borrowings - Schedule of Federal Home Loan Bank Advances by Branch of FHLB Bank (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Principal Amounts	$ 99,397	$ 29,462
Federal Home Loan Bank Advances One
Principal Amounts	$ 96,729	$ 12,262
Maturity Dates	2023	2022
Federal Home Loan Bank Advances One \| Minimum [Member]
Interest Rates	0.44%	0.00%
Federal Home Loan Bank Advances One \| Maximum [Member]
Interest Rates	4.38%	0.31%
Federal Home Loan Bank Advances Two
Principal Amounts	$ 800	$ 15,000
Maturity Dates	2024	2023
Interest Rates	0.00%
Federal Home Loan Bank Advances Two \| Minimum [Member]
Interest Rates		0.44%
Federal Home Loan Bank Advances Two \| Maximum [Member]
Interest Rates		0.45%
Federal Home Loan Bank Advances Three
Principal Amounts	$ 520	$ 800
Maturity Dates	2025	2024
Interest Rates	0.00%	0.00%
Federal Home Loan Bank Advances Four
Principal Amounts	$ 718	$ 520
Maturity Dates	2028	2025
Interest Rates	0.00%	0.00%
Federal Home Loan Bank Advances Five
Principal Amounts	$ 200	$ 250
Maturity Dates	2030	2028
Interest Rates	0.00%	0.00%
Federal Home Loan Bank Advances Six
Principal Amounts	$ 430	$ 200
Maturity Dates	2031	2030
Interest Rates	0.00%	0.00%
Federal Home Loan Bank Advances Seven
Principal Amounts		$ 430
Maturity Dates		2031
Interest Rates		0.00%

Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

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

- Definition

The year of maturity, when only one maturity date applies, for the advances from the Federal Home Loan Bank as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=123599511&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresDueDate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:gYearListItemType
Balance Type:	na
Period Type:	instant

- Definition

States the interest rate applicable to Federal Home Loan Bank advances outstanding as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123599511&loc=d3e64711-112823

+ Details

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

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesSixMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fsea_FederalHomeLoanBankAdvancesAxis=fsea_FederalHomeLoanBankAdvancesSevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: