Fair Value (Schedule of additional quantitative information about assets measured at fair value on a nonrecurring basis) (Details) - Nonrecurring basis $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 1,287	$ 1,063
Impaired loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		213	128
Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		1,074	935
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 1,287	$ 1,063
Impaired Loans, Valuation Technique [Extensible List]		rbkb:AppraisalOfCollateralMember	rbkb:AppraisalOfCollateralMember
Other Real Estate Owned, Valuation Technique [Extensible List]		Appraisal of collateral	Appraisal of collateral
Level 3 | Impaired loans
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 213	$ 128
Level 3 | Other real estate owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets held at fair value		$ 1,074	$ 935
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
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
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
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
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)	[2],[3]	0	0
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)	[2],[3]	6	6

[1] 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.

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

[3] Estimated costs to sell.