First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2022	Dec. 31, 2021
Income:
Expense									$ 5,674	$ 6,104
Net income (loss)	$ 2,228	$ 1,860	$ 1,415	$ 1,361	$ 1,711	$ 837	$ 953	$ 950	6,864	4,451
FUSB [Member]
Income:
Dividend income, First US Bank									3,463	1,570
Total income									3,463	1,570
Expense									1,320	1,055
Gain before equity in undistributed income of subsidiaries									2,143	515
Equity in undistributed income of subsidiaries									4,721	3,936
Net income (loss)									$ 6,864	$ 4,451