Regulatory Matters - Summary of Actual Capital Amounts and Ratios and Related Regulatory Requirements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Jan. 01, 2015
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier 1 Risk-Based Capital Ratio, Amount	$ 227,104	$ 178,645
Tier 1 Risk-Based Capital Ratio, Ratio	18.13%	15.81%
Tier 1 Risk-Based Capital Ratio, For Capital Adequacy Purposes Amount	$ 75,169	$ 67,813
Tier 1 Risk-Based Capital Ratio, For Capital Adequacy Purposes Ratio	6.00%	6.00%	4.00%
Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions Amount	$ 100,225	$ 90,417
Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions Ratio	8.00%	8.00%	6.00%
Total Risk-Based Capital Ratio, Amount	$ 239,013	$ 192,784
Total Risk-Based Capital Ratio, Ratio	19.08%	17.06%
Total Risk-Based Capital Ratio, For Capital Adequacy Purposes Amount	$ 100,225	$ 90,417
Total Risk-Based Capital Ratio, For Capital Adequacy Purposes Ratio	8.00%	8.00%
Total Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions Amount	$ 125,282	$ 113,021
Total Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions Ratio	10.00%	10.00%
RBB Bancorp
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier 1 Leverage Ratio, Amount	$ 232,521	$ 153,682
Tier 1 Leverage Ratio, Ratio	14.91%	11.00%
Common Equity Tier 1 Risk-Based Capital Ratio, Amount	$ 229,244	$ 150,786
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio	18.23%	13.30%
Tier 1 Risk-Based Capital Ratio, Amount	$ 232,521	$ 153,682
Tier 1 Risk-Based Capital Ratio, Ratio	18.49%	13.56%
Total Risk-Based Capital Ratio, Amount	$ 293,922	$ 217,244
Total Risk-Based Capital Ratio, Ratio	23.37%	19.17%
Royal Business Bank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier 1 Leverage Ratio, Amount	$ 227,104	$ 178,645
Tier 1 Leverage Ratio, Ratio	14.57%	12.81%
Tier 1 Leverage Ratio, For Capital Adequacy Purposes Amount	$ 62,363	$ 55,777
Tier 1 Leverage Ratio, For Capital Adequacy Purposes Ratio	4.00%	4.00%
Tier 1 Leverage Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions Amount	$ 77,953	$ 69,722
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	$ 227,104	$ 178,645
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio	18.13%	15.81%
Common Equity Tier 1 Risk-Based Capital Ratio, For Capital Adequacy Purposes Amount	$ 56,377	$ 50,860
Common Equity Tier 1 Risk-Based Capital Ratio, 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	$ 81,433	$ 73,464
Common Equity Tier 1 Risk-Based Capital Ratio, To Be Well-Capitalized Under Prompt Corrective Provisions Ratio	6.50%	6.50%

Common equity tier one risk based capital required for capital adequacy to risk weighted assets.

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Common equity tier one risk based capital required to be well capitalized to risk weighted assets.

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The amount of total capital as defined in the regulations.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Percentage of Common Equity Tier One Risk-based Capital to total risk-weighted assets, as defined by regulatory framework.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as adequately capitalized under regulatory framework for prompt corrective action.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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