Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	6 Months Ended
	Jul. 17, 2019 USD ($) shares	Jun. 30, 2020 USD ($) Service $ / shares shares	Dec. 31, 2019 USD ($) shares
Accounting Policies [Abstract]
Stock issued during period,initial public offer		2,676,740
Repurchase of share ESOP	238,473
Employee stock ownership price per share | $ / shares		$ 10.00
Plan of reorganization, description of equity securities issued or to be issued		In addition, as part of the Reorganization, the Company issued 3,345,925 shares of common stock to First Seacoast Bancorp, MHC (the “MHC”), the Bank’s parent mutual holding company, and 60,835 shares of common stock and $150,000 in cash to First Seacoast Community Foundation, Inc. (the “Foundation”), a charitable foundation formed in connection with the reorganization and dedicated to supporting charitable organizations operating in the Bank’s local community. The Company’s common stock began trading on the NASDAQ Capital Market under the symbol “FSEA” on July 17, 2019. Pursuant to the Plan of Reorganization, the Bank adopted an employee stock ownership plan (“ESOP”), which purchased 238,473 shares of common stock in the stock offering with the proceeds of a loan from the Company. As a result of the Reorganization, a total of 6,083,500 shares of common stock of the Company are issued and outstanding, of which 55% are issued to the MHC, 44% were sold to the Bank’s eligible members, the ESOP, and certain other persons in the stock offering, and 1% were contributed to the Foundation.
Stock issued to mutual holding company as part of reorganization	3,345,925
Stock issued during period shares to charitable organisations	60,835
Cash transferred to charitable foundation as part of reorganization | $	$ 150,000
ESOP,unearned compensation, shares		238,473
Common stock, shares, issued		6,083,500	6,083,500
Percentage of common stock shares offered to eligible members	55.00%
Percentage of common stock shares offered to charitable foundation	44.00%
Equity method investment ownership percentage	1.00%
Stock issuance cost | $	$ 1,600,000
Number of core services | Service		4
Assets under management | $		$ 49,600,000	$ 49,300,000
Emerging growth of company status		(i) the last day of the fiscal year of the Company during which it had total annual gross revenues of $1.07 billion (as adjusted for inflation) or more; (ii) the last day of the fiscal year of the Company following the fifth anniversary of the effective date of the Company’s initial public offering; (iii) the date on which the Company has, during the previous three-year period, issued more than $1.0 billion in non-convertible debt; or (iv) the date on which the Company is deemed to be a “large accelerated filer” under Securities and Exchange Commission regulations (generally, at least $700 million of voting and non-voting equity held by non-affiliates).