Condensed Financial Statements of Parent Company - Schedule of Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Other assets	$ 2,561	$ 1,666
Total assets	487,074	443,062
Financial Liabilities:
Other liabilities	1,520	1,411
Total liabilities	426,606	384,201
Stockholders' Equity:
Stockholders’ equity	60,468	58,861	$ 57,066
Total liabilities and stockholders' equity	487,074	443,062
First Seacoast Bancorp (Parent) Company
ASSETS
Cash held at First Seacoast Bank	9,785	10,118
Investment in First Seacoast Bank	48,477	46,467
Loan to First Seacoast Bank ESOP	2,105	2,180
Deferred tax asset	60	85
Other assets	43	18
Total assets	60,470	58,868
Financial Liabilities:
Other liabilities	2	7
Total liabilities	2	7
Stockholders' Equity:
Stockholders’ equity	60,468	58,861
Total liabilities and stockholders' equity	$ 60,470	$ 58,868