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Deposits - Deposit Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Deposits [Abstract]
NOW and demand deposits	$ 168,927	$ 161,859
Money market deposits	69,668	71,107
Savings deposits	84,524	85,511
Time deposits of greater than $250,000	21,887	18,822
Time deposits less than $250,000	135,027	116,909
Total deposits	$ 480,033	$ 454,208

X

- Definition

Deposits negotiable order of withdrawal and demand deposits.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations of less than $250000

+ References

No definition available.

+ Details

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

X

- Definition

Time deposits of greater than $250,000.

+ References

No definition available.

+ Details

Name:	fsea_TimeDepositsOfGreaterThan250000
Namespace Prefix:	fsea_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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

X

- References

No definition available.

+ Details

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

X

- Definition

Amount of deposits in accounts that offer many of the same services as checking accounts although transactions may be somewhat more limited, share some of the characteristics of a money market fund and are insured by the Federal government, also known as money market accounts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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

X

- Definition

Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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