Stock Based Compensation - Additional Information (Detail) - USD ($)							3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 02, 2024	Nov. 18, 2024	Nov. 18, 2023	Jun. 01, 2023	May 25, 2023	Nov. 18, 2021	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 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							7,678		7,678		15,354
Unearned compensation for ESOP							$ 4,608,000		$ 4,608,000		$ 4,858,000
Stock options granted, outstanding									249,144		280,500
Share price				$ 7.99							$ 9.29
Income tax (benefit) expense							(12,000)	$ (102,000)	$ 17,000	$ (464,000)
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	204,000,000
Income tax (benefit) expense							0	(27,000,000)	$ 0	(54,000,000)
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							220,000		$ 220,000		$ 335,000
Share based expense recognized							60,000	63,000	115,000	125,000
Income tax (benefit) expense							17,000	18,000	$ 33,000	36,000
Weighted average future recognition period									10 months 24 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							848,000		$ 848,000		$ 1,000,000
Stock awards unrecognized compensation expense							868,000		$ 868,000		$ 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		$ 174,000,000
Income tax (benefit) expense							(24,000,000)		$ (48,000,000)
Weighted average future recognition period									2 years 4 months 24 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		$ 181,000,000
Income tax (benefit) expense							$ (25,000,000)		$ (50,000,000)
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							$ 43,000	$ 36,000	$ 84,000	$ 68,000
Unearned compensation for ESOP							3,800,000		3,800,000		$ 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

First Seacoast Bancorp, Inc. 10-Q