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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, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018
Income: |  |  |  |  |  |  |  |  |  |
Expense |  |  |  |  |  |  |  |  | $ 6,499 | $ 5,965
Net income (loss) | $ 1,209 | $ 1,118 | $ 1,005 | $ 1,234 | $ 1,477 | $ 240 | $ 359 | $ 414 | 4,566 | 2,490
FUSB [Member] |  |  |  |  |  |  |  |  |  |
Income: |  |  |  |  |  |  |  |  |  |
Dividend income, First US Bank |  |  |  |  |  |  |  |  | 3,104 | 976
Total income |  |  |  |  |  |  |  |  | 3,104 | 976
Expense |  |  |  |  |  |  |  |  | 924 | 831
Gain before equity in undistributed income of subsidiaries |  |  |  |  |  |  |  |  | 2,180 | 145
Equity in undistributed income of subsidiaries |  |  |  |  |  |  |  |  | 2,386 | 2,345
Net income (loss) |  |  |  |  |  |  |  |  | $ 4,566 | $ 2,490

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