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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, 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

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