Short-Term Borrowings and Long-Term Debt - Short-term borrowings (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Short-Term Borrowings
Short term borrowings	$ 82,233,000	$ 114,300,000
Federal funds purchased \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Number of borrowing facilities \| item	2
Short term borrowings	$ 549,000	0
Federal funds purchased, facility one \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Maximum borrowing capacity	24,000,000
Federal funds purchased, facility two \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Maximum borrowing capacity	15,000,000
Federal Reserve discount window \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Maximum borrowing capacity	10,200,000
Short term borrowings	0	0
Short-term borrowing with interest rate 2.70% \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Short term borrowings	$ 79,900,000
Interest rate (as a percent)	2.70%
Short-term borrowing with interest rate 1.70% \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Short term borrowings	$ 1,800,000	$ 1,800,000
Interest rate (as a percent)	1.70%	1.70%
Duration of debt (in years)	4 years	4 years
Short-term borrowings with interest rate 2.62% \| Federal Home Loan Bank of Pittsburgh
Short-Term Borrowings
Short term borrowings		$ 112,500,000
Interest rate (as a percent)		2.62%

The number of borrowing facilities.

No definition available.

Name:	mrbk_NumberOfBorrowingFacilities
Namespace Prefix:	mrbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_ShortTermBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_ShortTermDebtLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_FederalFundsPurchasedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=srt_FederalHomeLoanBankOfPittsburghMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=mrbk_FederalFundsPurchasedFacilityOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=mrbk_FederalFundsPurchasedFacilityTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=mrbk_FederalReserveDiscountWindowBorrowingFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=mrbk_ShortTermDebtInterestRate2.70Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=mrbk_ShortTermDebtInterestRate1.70Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=mrbk_ShortTermDebtInterestRate2.62Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: