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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

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- Definition Brokered deposits. + References No definition available. + Details
Name: | fsea_BrokeredDeposits
Namespace Prefix: | fsea_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Reciprocal deposits. + References No definition available. + Details
Name: | fsea_ReciprocalDeposits
Namespace Prefix: | fsea_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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

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

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