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Regulatory Matters - Summary of Actual Capital Amounts and Ratios and Related Regulatory Requirements (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Jan. 01, 2015
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Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Ratio |  |  | 4.00%
Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Ratio |  |  | 6.00%
RBB Bancorp |  |  |
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |
Tier 1 Leverage Ratio, Amount | $ 353,572 | $ 321,407 |
Tier 1 Leverage Ratio, Ratio | 12.89% | 11.80% |
Common Equity Tier 1 Risk-Based Capital Ratio, Amount | $ 343,899 | $ 311,901 |
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio | 17.16% | 15.28% |
Tier 1 Risk-Based Capital Ratio, Amount | $ 353,572 | $ 321,407 |
Tier 1 Risk-Based Capital Ratio, Ratio | 17.65% | 15.74% |
Total Risk-Based Capital Ratio, Amount | $ 477,262 | $ 443,379 |
Total Risk-Based Capital Ratio, Ratio | 23.82% | 21.71% |
Royal Business Bank |  |  |
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |
Tier 1 Leverage Ratio, Amount | $ 417,036 | $ 370,304 |
Tier 1 Leverage Ratio, Ratio | 15.23% | 13.66% |
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Amount | $ 108,150 | $ 108,445 |
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio | 4.00% | 4.00% |
Tier 1 Leverage Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Amount | $ 135,187 | $ 135,556 |
Tier 1 Leverage Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Ratio | 5.00% | 5.00% |
Common Equity Tier 1 Risk-Based Capital Ratio, Amount | $ 417,036 | $ 370,304 |
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio | 20.87% | 18.17% |
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Amount | $ 89,127 | $ 91,722 |
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Ratio | 4.50% | 4.50% |
Common Equity Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Amount | $ 128,739 | $ 132,487 |
Common Equity Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Ratio | 6.50% | 6.50% |
Tier 1 Risk-Based Capital Ratio, Amount | $ 417,036 | $ 370,304 |
Tier 1 Risk-Based Capital Ratio, Ratio | 20.87% | 18.17% |
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Amount | $ 118,836 | $ 122,296 |
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Ratio | 6.00% | 6.00% |
Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Amount | $ 158,448 | $ 163,061 |
Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Ratio | 8.00% | 8.00% |
Total Risk-Based Capital Ratio, Amount | $ 436,677 | $ 388,569 |
Total Risk-Based Capital Ratio, Ratio | 21.86% | 19.07% |
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Amount | $ 158,448 | $ 163,061 |
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes, Ratio | 8.00% | 8.00% |
Total Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Amount | $ 198,061 | $ 203,826 |
Total Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions, Ratio | 10.00% | 10.00% |

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