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Fair Value Measurements and Disclosures - Valuation Techniques for Level 3 interest rate lock (Details) - Interest rate lock commitments - Level 3 $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset	$ 1,712		$ 1,712		$ 6,932
Valuation technique extensible list	us-gaap:ValuationTechniqueConsensusPricingModelMember		us-gaap:ValuationTechniqueConsensusPricingModelMember		us-gaap:ValuationTechniqueConsensusPricingModelMember
Measurement input extensible list	mrbk:LoanOriginationSucessRateMember		mrbk:LoanOriginationSucessRateMember		mrbk:LoanOriginationSucessRateMember
Net realized gains (losses)	$ (1,100)	$ 3,200	$ (5,500)	$ 7,200
Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1		1		1
Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	99		99		99
Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	87.26		87.26		83.08

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- Definition

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Value of input used to measure derivative asset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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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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