Fair Value (Schedule of additional quantitative information about assets measured at fair value on a nonrecurring basis) (Details) - Nonrecurring basis $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 1,569	$ 1,656
Impaired loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value			239
Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		1,382	1,417
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 1,569	$ 1,656
Impaired Loans, Valuation Technique [Extensible List]		rbkb:AppraisalOfCollateralMember	rbkb:AppraisalOfCollateralMember
Other Real Estate Owned, Valuation Technique [Extensible List]		Appraisal of collateral	Appraisal of collateral
Significant Unobservable Inputs (Level 3) | Impaired loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 187	$ 239
Significant Unobservable Inputs (Level 3) | Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 1,382	$ 1,417
Significant Unobservable Inputs (Level 3) | Appraisal adjustments | Minimum | Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, unobservable input (in percent)	[1],[2]	0	0
Significant Unobservable Inputs (Level 3) | Appraisal adjustments | Maximum | Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, unobservable input (in percent)	[1],[2]	20	20
Significant Unobservable Inputs (Level 3) | Appraisal of collateral | Appraisal adjustments | Minimum | Impaired loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, unobservable input (in percent)	[1],[2]	0	0
Significant Unobservable Inputs (Level 3) | Appraisal of collateral | Appraisal adjustments | Maximum | Impaired loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, unobservable input (in percent)	[1],[2]	20	20
Significant Unobservable Inputs (Level 3) | Appraisal of collateral | Liquidation expenses | Minimum | Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, unobservable input (in percent)	[1],[3]	0	0
Significant Unobservable Inputs (Level 3) | Appraisal of collateral | Liquidation expenses | Maximum | Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned, unobservable input (in percent)	[1],[3]	6	6

[1] (1)Fair value is generally through independent appraisals of the underlying collateral that generally include various level 3 inputs which are not identifiable.

[2] (2)Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated liquidation expenses. The range of liquidation expenses and other appraisal adjustments are presented as a percent of the appraised value.

[3] (3)Estimated costs to sell.