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v3.23.2

Deposits - Additional information (Detail) - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
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Time Deposits [Member] |  |
Brokered time deposits | $ 13.6 | $ 18.1

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