Securities Available-for-Sale - Additional Information (Detail) - Security	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Available For Sale Securities [Abstract]
Number of holdings more than ten percentage of fair value of securities issued by single issuer	0	0

Number of holdings more than ten percentage of fair value of securities issued by single issuer.

No definition available.

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