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First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2024	Dec. 31, 2023
Income:
Total income									$ 1,232	$ 1,197
Expense									4,754	5,142
Net income (loss)	$ 1,714	$ 2,222	$ 2,127	$ 2,107	$ 2,277	$ 2,113	$ 2,023	$ 2,072	8,170	8,485
FUSB [Member]
Income:
Dividend income, First US Bank									5,541	3,460
Total income									5,541	3,460
Expense									1,450	1,362
Gain before equity in undistributed income of subsidiaries									4,091	2,098
Equity in undistributed income of subsidiaries									4,079	6,387
Net income (loss)									$ 8,170	$ 8,485

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