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Deposits - Additional information (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Time Deposits [Member]
Brokered time deposits	$ 18.1	$ 0.0

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

- Details

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