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
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset	$ 4,021		$ 504
Valuation technique extensible list	us-gaap:ValuationTechniqueConsensusPricingModelMember		us-gaap:ValuationTechniqueConsensusPricingModelMember
Measurement input extensible list	mrbk:LoanOriginationSucessRateMember		mrbk:LoanOriginationSucessRateMember
Unrealized losses	$ 3,300	$ 19
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		93.41