First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Income:
Expense									$ 6,485	$ 6,499
Net income (loss)	$ 1,045	$ 411	$ 404	$ 847	$ 1,209	$ 1,118	$ 1,005	$ 1,234	2,707	4,566
FUSB [Member]
Income:
Dividend income, First US Bank									2,167	3,104
Total income									2,167	3,104
Expense									1,046	924
Gain before equity in undistributed income of subsidiaries									1,121	2,180
Equity in undistributed income of subsidiaries									1,586	2,386
Net income (loss)									$ 2,707	$ 4,566