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Deposits - Additional information (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Reciprocal deposits	$ 9,100	$ 6,000
Deposits from related parties	12,600	11,600
Time Deposits [Member]
Brokered time deposits	75,000	63,100
Savings Deposits [Member]
Brokered time deposits	$ 22,000	$ 22,100

X

- Definition

Brokered deposits.

+ References

No definition available.

+ Details

Name:	fsea_BrokeredDeposits
Namespace Prefix:	fsea_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Reciprocal deposits.

+ References

No definition available.

+ Details

Name:	fsea_ReciprocalDeposits
Namespace Prefix:	fsea_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of deposits held by the entity for a related party (entity, shareholder, employee).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

+ Details

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

X

- Details

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

X

- Details

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