Condensed Financial Statements of Parent Company - Schedule of Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
ASSETS
Other assets	$ 9,171	$ 2,561
Total assets	537,424	487,074
LIABILITIES
Other liabilities	3,559	1,520
Total liabilities	488,087	426,606
STOCKHOLDERS' EQUITY
Stockholders’ equity	49,337	60,468	$ 58,861
Total liabilities and stockholders' equity	537,424	487,074
First Seacoast Bancorp (Parent) Company
ASSETS
Cash held at First Seacoast Bank	9,346	9,785
Investment in First Seacoast Bank	37,925	48,477
Loan to First Seacoast Bank ESOP	2,025	2,105
Deferred tax asset		60
Other assets	41	43
Total assets	49,337	60,470
LIABILITIES
Other liabilities		2
Total liabilities		2
STOCKHOLDERS' EQUITY
Stockholders’ equity	49,337	60,468
Total liabilities and stockholders' equity	$ 49,337	$ 60,470