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Stock Based Compensation - Additional Information (Detail) - USD ($) |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 02, 2024 | Nov. 18, 2024 | Nov. 18, 2023 | Jun. 01, 2023 | May 25, 2023 | Nov. 18, 2021 | Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of shares committed to be released each year, ESOP |  |  |  |  |  |  | 11,517 |  | 11,517 |  | 15,354
Unearned compensation for ESOP |  |  |  |  |  |  | $ 4,483,000 |  | $ 4,483,000 |  | $ 4,858,000
Stock options granted, outstanding |  |  |  |  |  |  |  |  | 249,144 |  | 280,500
Share price |  |  |  | $ 7.99 |  |  |  |  |  |  | $ 9.29
Income tax (benefit) expense |  |  |  |  |  |  | 596,000 | $ 640,000 | $ 613,000 | $ 176,000 |
Restricted stock awards granted |  |  |  | 2,478 |  |  |  |  |  |  | 112,200
Fair value related to grant | $ 1,000,000 |  |  | $ 20,000 |  |  |  |  |  |  |
Restricted Stock Awards [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restricted stock awards time-vest |  | 50.00% | 50.00% |  |  |  |  |  |  |  |
Minimum [Member] | Restricted Stock Awards [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Issued stock option granted, term |  |  |  |  |  |  |  |  | 3 years |  |
Directors and Members [Member] | Restricted Stock Awards [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Restricted Stock Award, |  |  |  |  |  | 98,850 |  |  |  |  |
Share price |  |  |  |  |  |  |  |  | $ 11.95 |  |
Total fair value related to stock options granted |  |  |  |  |  |  |  |  | $ 1,200,000 |  |
2021 Equity Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common stock shares issued during period, new issues |  |  |  |  |  |  |  |  | 348,801 |  |
Issued stock option granted, term |  |  |  |  |  |  |  |  | 10 years |  |
Stock options granted, outstanding |  |  |  |  | 249,144 |  |  |  |  |  |
2021 Equity Incentive Plan [Member] | Restricted Stock Awards [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share based expense recognized |  |  |  |  |  |  | 0 | 102,000,000 | $ 0 | 306,000,000 |
Income tax (benefit) expense |  |  |  |  |  |  | 0 | (27,000,000) | $ 0 | (81,000,000) |
2021 Equity Incentive Plan [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Exercise of Option |  |  |  |  |  |  |  |  | 249,144 |  |
Stock Issued During Period, Shares, Restricted Stock Award, |  |  |  |  |  |  |  |  | 99,657 |  |
2021 Equity Incentive Plan [Member] | Director [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unearned compensation for ESOP |  |  |  |  |  |  | 160,000 |  | $ 160,000 |  | $ 335,000
Share based expense recognized |  |  |  |  |  |  | 60,000 | 62,000 | 175,000 | 187,000 |
Income tax (benefit) expense |  |  |  |  |  |  | 15,000 | 16,000 | $ 47,000 | 50,000 |
Weighted average future recognition period |  |  |  |  |  |  |  |  | 7 months 6 days |  | 1 year 4 months 24 days
2024 Equity Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options unrecognized compensation expense |  |  |  |  |  |  | 757,000 |  | $ 757,000 |  | $ 1,000,000
Stock awards unrecognized compensation expense |  |  |  |  |  |  | 781,000 |  | $ 781,000 |  | $ 1,000,000
Common stock shares issued during period, new issues |  |  |  |  |  |  |  |  | 392,700 |  |
Stock options granted, outstanding | 280,500 |  |  |  |  |  |  |  |  |  |
Share based expense recognized |  |  |  |  |  |  | 87,000,000 |  | $ 261,000,000 |  |
Income tax (benefit) expense |  |  |  |  |  |  | (24,000,000) |  | $ (72,000,000) |  |
Weighted average future recognition period |  |  |  |  |  |  |  |  | 2 years 1 month 6 days |  | 2 years 10 months 24 days
Restricted stock awards granted | 112,200 |  |  |  |  |  |  |  |  |  |
2024 Equity Incentive Plan [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Exercise of Option |  |  |  |  |  |  |  |  | 280,500 |  |
Stock Issued During Period, Shares, Restricted Stock Award, |  |  |  |  |  |  |  |  | 112,200 |  |
2024 Equity Incentive Plan [Member] | Director [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share price | $ 9.29 |  |  |  |  |  |  |  |  |  |
Share based expense recognized |  |  |  |  |  |  | 91,000,000 |  | $ 272,000,000 |  |
Income tax (benefit) expense |  |  |  |  |  |  | $ (25,000,000) |  | $ (75,000,000) |  |
First Seacoast Bank Employee Stock Ownership Plan [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of shares committed to be released each year, ESOP |  |  |  |  |  |  | 15,354 |  | 15,354 |  |
Employee stock option compensation recognized |  |  |  |  |  |  | $ 44,000 | $ 35,000 | $ 128,000 | $ 103,000 |
Unearned compensation for ESOP |  |  |  |  |  |  | 3,700,000 |  | 3,700,000 |  | $ 3,800,000
Employee stock option unallocated share fair value |  |  |  |  |  |  | 3,900,000 |  | $ 3,900,000 |  | 3,500,000
Employee stock option plan [Member] |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock repurchase |  |  |  |  |  |  |  |  | 423,715 |  |
Percentage of purchase price common stock |  |  |  |  |  |  |  |  | 100.00% |  |
Remaining principal balance of debt |  |  |  |  |  |  | $ 4,100,000 |  | $ 4,100,000 |  | $ 4,100,000

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- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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