Loans and Allowance for Loan Losses - Non Accrual Loans and Loans Past Due 90 Days Or More and Still Accruing (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Loan portfolio past due loans
Nonaccrual	$ 2,548	$ 1,202
Total Loans Greater than 90 Days & Accruing	592	685
Residential real estate
Loan portfolio past due loans
Nonaccrual	60	303
Total Loans Greater than 90 Days & Accruing	475	578
Number of residential loans	0	0
Commercial and commercial real estate
Loan portfolio past due loans
Nonaccrual	$ 2,060	$ 899
Number of troubled debt restructuring \| item	1	0
Outstanding balance	$ 2,000
Term	1 year
Variable rate	1
Agricultural production and real estate
Loan portfolio past due loans
Nonaccrual	$ 282
Total Loans Greater than 90 Days & Accruing	117	$ 107
Consumer and margin loans
Loan portfolio past due loans
Nonaccrual	$ 146

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

No definition available.

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

The amount of residential loans in process of foreclosure.

+ References

No definition available.

+ Details

Name:	mbin_ResidentialLoansInProcessOfForeclosure
Namespace Prefix:	mbin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

The amount of the outstanding recorded investment related to 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 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded investment in financing receivables that are 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_FinancingReceivableRecordedInvestmentPastDueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

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