Condensed Financial Statements of Parent Company - Schedule of Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities:
Net income	$ 2,621	$ 1,079
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred tax expense (benefit)	309	(337)
(Increase) decrease in other assets	(771)	488
Net cash provided by operating activities	2,421	1,421
Cash flows from investing activities:
Net cash used by investing activities	(43,498)	(31,903)
Cash flows from financing activities:
Net cash provided by financing activities	41,719	32,469
Net change in cash and cash equivalents	642	1,987
Cash and cash equivalents at beginning of year	5,996	4,009
Cash and cash equivalents at end of year	6,638	5,996
First Seacoast Bancorp (Parent) Company
Cash flows from operating activities:
Net income	2,621	1,079
Adjustments to reconcile net income to net cash provided by operating activities:
Undistributed net income of First Seacoast Bank	(2,509)	(962)
Deferred tax expense (benefit)	25	18
(Increase) decrease in other assets	(25)	172
(Decrease) increase in other liabilities	(5)	7
Net cash provided by operating activities	107	314
Cash flows from investing activities:
Principal payments received on ESOP	75	72
Net cash used by investing activities	75	72
Cash flows from financing activities:
Treasury stock purchases	(515)	(233)
Net cash provided by financing activities	(515)	(233)
Net change in cash and cash equivalents	(333)	153
Cash and cash equivalents at beginning of year	10,118	9,965
Cash and cash equivalents at end of year	$ 9,785	$ 10,118