Percentage of Common stock repurchase

No definition available.

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- Definition

It represents the number of treasury stock shares outstanding.

+ References

No definition available.

+ Details

Name:	fsea_TreasuryStockSharesOutstanding
Namespace Prefix:	fsea_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Vested restricted stock award surrendered in lieu of a cash payment.

+ References

No definition available.

+ Details

Name:	fsea_VestedRestrictedStockAwardSurrenderedInLieuOfACashPayment
Namespace Prefix:	fsea_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_EquityClassOfTreasuryStockLineItems
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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The remaining number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix:
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fsea_StockholdersOtherThanFirstSeacoastBancorpMutualHoldingCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Treasury Stock - Additional Information (Detail) - shares	Dec. 31, 2022	Nov. 18, 2022	Dec. 31, 2021	Sep. 23, 2020
Equity Class Of Treasury Stock [Line Items]
Vested restricted stock award surrendered in lieu of cash payment		593
Treasury Stock, Shares outstanding	137,472		78,433
Common Stock [Member]
Equity Class Of Treasury Stock [Line Items]
Percentage of repurchase of Common Stock				2.20%
Number of shares authorized for repurchase	136,879		78,433
Common Stock [Member] \| Maximum [Member]
Equity Class Of Treasury Stock [Line Items]
Number of shares authorized for repurchase				136,879
Common Stock [Member] \| Stockholders other than First Seacoast Bancorp, MHC [Member]
Equity Class Of Treasury Stock [Line Items]
Equity method investment ownership percentage				5.00%