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
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%