First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020
Income:
Expense									$ 6,104	$ 6,485
Net income (loss)	$ 1,711	$ 837	$ 953	$ 950	$ 1,045	$ 411	$ 404	$ 847	4,451	2,707
FUSB [Member]
Income:
Dividend income, First US Bank									1,570	2,167
Total income									1,570	2,167
Expense									1,055	1,046
Gain before equity in undistributed income of subsidiaries									515	1,121
Equity in undistributed income of subsidiaries									3,936	1,586
Net income (loss)									$ 4,451	$ 2,707