First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash on deposit	$ 94,415	$ 57,030
Other assets	8,862	9,739
Total assets	890,511	788,738
Other liabilities	11,312	9,766
Shareholders’ equity	86,678	84,748	$ 79,437
Total liabilities and shareholders’ equity	890,511	788,738
FUSB [Member]
Cash on deposit	419	476
Investment in subsidiaries	86,102	84,186
Other assets	104	246
Total assets	86,625	84,908
Other liabilities	53	160
Shareholders’ equity	86,678	84,748
Total liabilities and shareholders’ equity	$ 86,625	$ 84,908