CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - shares	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Statement of Stockholders' Equity [Abstract]
Number of shares committed to be released each year,ESOP	3,839	3,839	11,517	11,517

The number of shares committed to the ESOP plan, but not allocated or contributed to the plan yet. The ESOP documents typically define the period of service to which the shares relate. ESOP shares are released to compensate employees directly, to settle employer liabilities for other employee benefits, and to replace dividends on allocated shares that are used for debt service.

No definition available.

Name:	fsea_EmployeeStockOwnershipPlanESOPNumberOfCommittedToBeReleasedSharesDuringPeriod
Namespace Prefix:	fsea_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration