The fixed interest rate on the financing receivable.

No definition available.

Name:	mbin_FinancingReceivableRecordedInvestmentFixedRate
Namespace Prefix:	mbin_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the financing receivables.

+ References

No definition available.

+ Details

Name:	mbin_FinancingReceivableRecordedInvestmentSpreadVariableRate
Namespace Prefix:	mbin_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

The period for financing receivable recorded investment.

+ References

No definition available.

+ Details

Name:	mbin_FinancingReceivableRecordedInvestmentTerm
Namespace Prefix:	mbin_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that are in the process of foreclosure.

+ References

No definition available.

+ Details

Name:	mbin_MortgageLoansInProcessOfForeclosureNumberOfContracts
Namespace Prefix:	mbin_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=mbin_ConversionOfFinancingTermsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Loan Losses - Troubled Debt and Modifications (Details) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2018 item	Sep. 30, 2017 item	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Troubled debt and modifications
Number of loans restructured during the last twelve months that defaulted during the period \| item	0	0
Commercial and commercial real estate \| Extended Maturity
Troubled debt and modifications
Recorded investment, before modification				$ 2.0
Recorded investment, after modification			$ 2.0
Term			1 year
Variable rate (as a percent)			1
Commercial and commercial real estate \| Modification of terms
Troubled debt and modifications
Recorded investment, before modification			$ 2.0
Recorded investment, after modification			$ 2.0
Fixed rate (as a percent)			6.00%
Number of troubled debt restructuring \| item			1
Residential real estate
Troubled debt and modifications
Number of loans in the process of foreclosure \| item			0	0