Accounting policies.

No definition available.

Name:	mc_AccountingPoliciesLineItems
Namespace Prefix:	mc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum total age and consecutive period of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited, in PnYnMnDTnHnMnS' format.

+ References

No definition available.

+ Details

Name:	mc_EquityBasedCompensationMinimumAgeAndConsecutivePeriodOfServiceOfRetiringEmployeesToNotForfeitAwardsGranted
Namespace Prefix:	mc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum age of retiring employees required so that certain qualifying awards granted during employment will not be forfeited, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	mc_EquityBasedCompensationMinimumAgeOfRetiringEmployeeToNotForfeitCertainQualifyingAwardsGranted
Namespace Prefix:	mc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited, in PnYnMnDTnHnMnS' format, for example,

+ References

No definition available.

+ Details

Name:	mc_EquityBasedCompensationMinimumConsecutivePeriodsOfServiceOfRetiringEmployeeToNotForfeitCertainQualifyingAwardsGranted
Namespace Prefix:	mc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Income taxes.

+ References

No definition available.

+ Details

Name:	mc_IncomeTaxesAbstract
Namespace Prefix:	mc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Installment period.

+ References

No definition available.

+ Details

Name:	mc_InstallmentPeriod
Namespace Prefix:	mc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the remaining percentage of cash savings realized by the reporting entity under the tax receivable agreement.

+ References

No definition available.

+ Details

Name:	mc_TaxReceivableAgreementPaymentPercentageOfCashSavingsRealizedByReportingEntity
Namespace Prefix:	mc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the payment percentage of cash savings payable to eligible managing directors for any amounts realized under tax receivable agreement.

+ References

No definition available.

+ Details

Name:	mc_TaxReceivableAgreementPaymentPercentageOfCashSavingsRealizedToEligibleManagingDirectors
Namespace Prefix:	mc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The carrying amount of capitalized computer software costs net of accumulated amortization as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

+ Details

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

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631418-115840

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section 45
-Paragraph 21
-URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631419-115840

+ Details

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

- Definition

Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ 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 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of the cumulative effect on retained earnings before related income tax effect.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22663-107794

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122869-111746

+ Details

Name:	us-gaap_CumulativeEffectOnRetainedEarningsBeforeTax1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of the cumulative effect on retained earnings net of related income tax effect.

+ References

+ Details

Name:	us-gaap_CumulativeEffectOnRetainedEarningsNetOfTax1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of the related income tax effect of a cumulative effect on retained earnings.

+ References

+ Details

Name:	us-gaap_CumulativeEffectOnRetainedEarningsTax1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The net amount of operating interest income (expense).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.10)
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

+ Details

Name:	us-gaap_InterestIncomeExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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

- Definition

Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 10B
-URI http://asc.fasb.org/extlink&oid=116821951&loc=SL37586934-109318

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319

+ Details

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

- Details

Name:	us-gaap_PlanNameAxis=mc_IssuedPriorToDecember12016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=mc_IssuedOnOrAfterDecember12016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201602Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForChangeInAccountingPrincipleAxis=mc_AdjustmentForPrepaidRentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForChangeInAccountingPrincipleAxis=mc_AdjustmentForAccruedRentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201815Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=mc_OfficeEquipmentAndFurnitureAndFixturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 01, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jan. 01, 2019
Accounting Policies [Line Items]
Cash		$ 26,724	$ 48,113	$ 26,724	$ 48,113	$ 71,798
Cash equivalents		66,226	60,247	66,226	60,247	96,014
Account receivables		19,781		19,781		19,879
Interest Income		198	245	386	520
Assets and Liabilities, Lessee
Operating Lease, Liability		194,903		194,903		197,625
Operating lease, right-of-use asset		182,625		$ 182,625		$ 190,763
Percentage of tax benefits payable to partners under tax receivable agreement				85.00%
Remaining percentage of cash savings realized by the Company (as a percent)				15.00%
Income Taxes
Unrecognized tax benefits		0	$ 0	$ 0	$ 0
Issued Prior to December 1, 2016
Revenue and Expense Recognition and Equity-Based Compensation
Minimum age of retiring employees required so that certain qualifying awards granted during employment will not be forfeited				54 years
Consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited				8 years
Issued on or After December 1, 2016
Revenue and Expense Recognition and Equity-Based Compensation
Minimum age of retiring employees required so that certain qualifying awards granted during employment will not be forfeited				56 years
Consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited				5 years
Minimum total age and consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited				65 years
ASU 2016-13
Accounting Policies [Line Items]
Cumulative effect on retained earnings, before tax	$ (459)
Cumulative effect on retained earnings, tax	95
Cumulative effect on retained earnings, net	$ (364)
ASU 2016-02
Assets and Liabilities, Lessee
Operating Lease, Liability							$ 63,252
Operating lease, right-of-use asset							63,252
ASU 2016-02 \| Adjustment For Prepaid Rent
Assets and Liabilities, Lessee
Operating lease, right-of-use asset							1,666
ASU 2016-02 \| Adjustment For Accrued Rent
Assets and Liabilities, Lessee
Operating lease, right-of-use asset							$ 7,139
ASU 2018-15
Assets and Liabilities, Lessee
Capitalized implementation cost of software		$ 1,512		$ 1,512
Minimum
Accounting Policies [Line Items]
Installment Period				3 years
Minimum \| Office Equipment and Furniture and Fixtures
Assets and Liabilities, Lessee
Useful lives				3 years
Maximum
Accounting Policies [Line Items]
Installment Period				4 years
Maximum \| Office Equipment and Furniture and Fixtures
Assets and Liabilities, Lessee
Useful lives				7 years