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Regulatory Matters - Additional Information (Details) - USD ($) |  |  |  |  |  | 9 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Jan. 01, 2019 | Jan. 01, 2018 | Jan. 02, 2017 | Jan. 02, 2016 | Jan. 01, 2015 | Sep. 30, 2017 | Dec. 31, 2016
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |  |  |  |  |
Common equity Tier 1 capital to risk-weighted assets ratio with minimum for capital adequacy |  |  |  |  | 4.50% |  |
Common equity Tier 1 capital to risk-weighted assets ratio with minimum for prompt corrective action purposes |  |  |  |  | 6.50% |  |
Tier 1 capital to risk-weighted assets ratio with minimum for capital adequacy |  |  |  |  | 4.00% | 6.00% | 6.00%
Increased Tier 1 capital to risk-weighted assets ratio with minimum for capital adequacy |  |  |  |  | 6.00% |  |
Tier 1 capital to risk-weighted assets ratio with minimum for prompt corrective action purposes |  |  |  |  | 6.00% | 8.00% | 8.00%
Increased Tier 1 capital to risk-weighted assets ratio with minimum for prompt corrective action purposes |  |  |  |  | 8.00% |  |
Capital conservation buffer |  |  | 2.50% | 0.625% | 2.50% |  |
Capital conservation buffer phased-in on pro rata basis period |  |  |  | 4 years |  |  |
Scenario, Forecast |  |  |  |  |  |  |
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |  |  |  |  |
Capital conservation buffer | 2.50% | 2.50% |  |  |  |  |
Royal Business Bank |  |  |  |  |  |  |
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |  |  |  |  |
Capital conservation buffer |  |  |  |  |  | 11.08% |
RBB Bancorp |  |  |  |  |  |  |
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |  |  |  |  |
Capital conservation buffer |  |  |  |  |  | 12.49% |
Minimum |  |  |  |  |  |  |
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  | $ 1,000,000,000
Required assets to liabilities ratio after dividend effect |  |  |  |  |  | 125.00% |

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- Definition Tier one risk based common equity capital required for capital adequacy to risk weighted assets. + References No definition available. + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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