Note 17 - Regulatory Matters - Summary of Company and Bank Continue to Exceed Regulatory Capital Minimum Requirements and Bank Continues to Exceed Well Capitalized Standards (Details) $ in Thousands	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
RBB Bancorp [Member]
Tier 1 Leverage Ratio, Amount	$ 469,453	$ 446,776
Tier 1 Leverage Ratio, Ratio	0.1168	0.1167
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 160,744	$ 153,116
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.04	0.04
Common Equity Tier 1 Risk-Based Capital Ratio, Amount	$ 454,569	$ 432,056
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio	0.1765	0.1603
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 115,916	$ 121,291
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1 Risk-Based Capital Ratio, Amount	$ 469,453	$ 446,776
Tier 1 Risk-Based Capital Ratio, Ratio	0.1822	0.1658
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 154,554	$ 161,722
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.06	0.06
Total Risk-Based Capital Ratio, Amount	$ 675,805	$ 654,159
Total Risk-Based Capital Ratio, Ratio	0.2624	0.2427
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 206,072	$ 215,629
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.08	0.08
Royal Business Bank [Member]
Tier 1 Leverage Ratio, Amount	$ 561,797	$ 569,071
Tier 1 Leverage Ratio, Ratio	0.1399	0.1489
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 160,666	$ 152,900
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.04	0.04
Tier 1 Leverage Ratio, Minimum Required Plus Capital Conservation Buffer Fully Phased-In Amount	$ 200,833	$ 191,124
Tier 1 Leverage Ratio, Minimum Required Plus Capital Conservation Buffer Fully Phased-In Ratio	5.00%	5.00%
Common Equity Tier 1 Risk-Based Capital Ratio, Amount	$ 561,797	$ 569,071
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio	0.2183	0.2114
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 115,791	$ 121,110
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, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Amount	$ 167,254	$ 174,937
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Ratio	0.065	0.065
Tier 1 Risk-Based Capital Ratio, Amount	$ 561,797	$ 569,071
Tier 1 Risk-Based Capital Ratio, Ratio	0.2183	0.2114
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 154,389	$ 161,481
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.06	0.06
Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Amount	$ 205,851	$ 215,307
Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Ratio	8.00%	8.00%
Total Risk-Based Capital Ratio, Amount	$ 594,096	$ 602,819
Total Risk-Based Capital Ratio, Ratio	0.2309	0.224
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 205,851	$ 215,307
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.08	0.08
Total Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Amount	$ 257,314	$ 269,134
Total Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Ratio	10.00%	10.00%