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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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Capital conservation buffer.

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Capital conservation buffer phased-in on pro rata basis period.

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- Definition

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

+ References

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- Definition

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

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Required assets to liabilities ratio after dividend effect.

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Tier one risk based common equity capital required for capital adequacy to risk weighted assets.

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Tier one risk based common equity capital required to be well capitalized to risk weighted assets.

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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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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

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
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-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

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- Definition

The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

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