Financial Instruments with Off-Balance Sheet Risk, Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Financial Instruments With Off Balance Sheet Risk Commitment And Contingencies [Abstract]
Lease expiration date	2022-06
Future lease payments	$ 16,000
Operating lease expense	$ 32,000	$ 32,000

Financial instruments with off balance sheet risk, commitment and contingencies.

No definition available.

Name:	fsea_FinancialInstrumentsWithOffBalanceSheetRiskCommitmentAndContingenciesAbstract
Namespace Prefix:	fsea_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Lease expiration date in month and year (YYYY-MM).

+ References

No definition available.

+ Details

Name:	fsea_LeaseExpirationDateInMonthAndYear
Namespace Prefix:	fsea_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating lease expense. Excludes sublease income.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123391704&loc=SL77918638-209977

+ Details

Name:	us-gaap_OperatingLeaseExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123406913&loc=d3e41502-112717

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=123403562&loc=d3e38371-112697

+ Details

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