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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Fair Value Measurements and Disclosures - Valuation Techniques for Level 3 interest rate lock (Details) - Interest rate lock commitments $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset	$ 283		$ 283
Valuation technique extensible list	us-gaap:ValuationTechniqueConsensusPricingModelMember		us-gaap:ValuationTechniqueConsensusPricingModelMember
Measurement input extensible list	mrbk:LoanOriginationSucessRateMember		mrbk:LoanOriginationSucessRateMember
Gain in fair value of derivative assets	$ 101	$ 138	$ 67	$ 114
Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1		1
Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	99		99
Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	91.70		91.70