XML 59 R52.htm IDEA: XBRL DOCUMENT
v3.25.3

Loans and Allowance for Credit Losses on Loans - Summary of Provision (Release) For Credit Losses (Details) - USD ($) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Receivables [Abstract] |  |  |  |
Loans |  | $ 20,000 | $ 30,000 | $ 80,000
Off-balance sheet credit exposures | $ 3,000 | (4,000) | 20,000 | (100,000)
Total provision (release) for credit losses | $ 3,000 | $ 16,000 | $ 50,000 | $ (20,000)

X
- Definition Provision (benefit) for loan losses. + References No definition available. + Details
Name: | fsea_ProvisionBenefitForLoanLosses
Namespace Prefix: | fsea_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Provision release for credit losses. + References No definition available. + Details
Name: | fsea_ProvisionReleaseForCreditLosses
Namespace Prefix: | fsea_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-13 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 22 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147476148/220-40-50-22 + Details
Name: | us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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