The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mcb_MultifamilyRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

LOANS (Schedule of Loans by Impairment Method) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	$ 204		$ 86
Collectively evaluated for impairment, Allowance for loan losses	18,289		14,801
Total ending allowance balance	18,493	$ 17,463	14,887	$ 15,075	$ 13,909	$ 11,815
Individually evaluated for impairment, Loans	2,873		6,089
Collectively evaluated for impairment, Loans	1,697,962		1,414,877
Total ending loan balance	1,700,835		1,420,966
Real estate
Loans and Leases Receivable Disclosure [Line Items]
Total ending loan balance	1,217,358		1,036,370
Real estate \| Commercial
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	0		0
Collectively evaluated for impairment, Allowance for loan losses	8,551		7,136
Total ending allowance balance	8,551	8,139	7,136	7,124	6,487	5,206
Individually evaluated for impairment, Loans	1,522		2,368
Collectively evaluated for impairment, Loans	891,555		781,377
Total ending loan balance	893,077		783,745
Real estate \| Construction
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	0		0
Collectively evaluated for impairment, Allowance for loan losses	671		519
Total ending allowance balance	671	666	519	524	557	409
Individually evaluated for impairment, Loans	0		0
Collectively evaluated for impairment, Loans	47,711		36,960
Total ending loan balance	47,711		36,960
Real estate \| Multifamily
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	0		0
Collectively evaluated for impairment, Allowance for loan losses	1,700		1,156
Total ending allowance balance	1,700	1,557	1,156	992	958	620
Individually evaluated for impairment, Loans	0		0
Collectively evaluated for impairment, Loans	253,775		190,097
Total ending loan balance	253,775		190,097
Real estate \| One to four family
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	0		9
Collectively evaluated for impairment, Allowance for loan losses	336		129
Total ending allowance balance	336	380	138	87	85	109
Individually evaluated for impairment, Loans	1,088		3,566
Collectively evaluated for impairment, Loans	21,707		22,002
Total ending loan balance	22,795		25,568
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	0		0
Collectively evaluated for impairment, Allowance for loan losses	6,326		5,578
Total ending allowance balance	6,326	6,053	5,578	6,003	5,560	5,364
Individually evaluated for impairment, Loans	0		0
Collectively evaluated for impairment, Loans	366,739		340,001
Total ending loan balance	366,739		340,001
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Individually evaluated for impairment, Allowance for loan losses	204		77
Collectively evaluated for impairment, Allowance for loan losses	705		283
Total ending allowance balance	909	$ 668	360	$ 345	$ 262	$ 107
Individually evaluated for impairment, Loans	263		155
Collectively evaluated for impairment, Loans	116,475		44,440
Total ending loan balance	$ 116,738		$ 44,595