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CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - shares | 3 Months Ended | 9 Months Ended
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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

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- Definition 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. + References No definition available. + Details
Name: | fsea_EmployeeStockOwnershipPlanESOPNumberOfCommittedToBeReleasedSharesDuringPeriod
Namespace Prefix: | fsea_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_StatementOfStockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration