Securities Available-for-Sale - Additional Information (Detail) - Security	3 Months Ended	12 Months Ended
Mar. 31, 2022	Dec. 31, 2021
Available For Sale Securities [Abstract]
Number of holdings of securities aggregate carrying value exceeded ten percentage of stockholders' equity	0	0

Number of holdings of securities aggregate carrying value exceeded ten percentage of equity.

No definition available.

Name:	fsea_NumberOfHoldingsOfSecuritiesAggregateCarryingValueExceededTenPercentageOfEquity
Namespace Prefix:	fsea_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration