Condensed Financial Statements of Parent Company - Schedule of Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	$ (565)	$ 2,621
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Deferred tax expense	(296)	309
Decrease (increase) in other assets	(1,483)	(771)
Net cash provided by operating activities	973	2,421
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash used by investing activities	(58,073)	(43,498)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net cash provided by financing activities	58,712	41,719
Net change in cash and cash equivalents	1,612	642
Cash and cash equivalents at beginning of year	6,638	5,996
Cash and cash equivalents at end of year	8,250	6,638
First Seacoast Bancorp (Parent) Company
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	(565)	2,621
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Undistributed net loss (income) of First Seacoast Bank	609	(2,509)
Deferred tax expense	60	25
Decrease (increase) in other assets	2	(25)
Decrease in other liabilities	(2)	(5)
Net cash provided by operating activities	104	107
CASH FLOWS FROM INVESTING ACTIVITIES:
Principal payments received on ESOP	80	75
Net cash used by investing activities	80	75
CASH FLOWS FROM FINANCING ACTIVITIES:
Treasury stock purchases	(623)	(515)
Net cash provided by financing activities	(623)	(515)
Net change in cash and cash equivalents	(439)	(333)
Cash and cash equivalents at beginning of year	9,785	10,118
Cash and cash equivalents at end of year	$ 9,346	$ 9,785