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

After-tax effect of change in accounting principle.

+ References

No definition available.

+ Details

Name:	mc_NewAccountingPronouncementOrChangeInAccountingPrincipleAfterTaxEffectOfChange
Namespace Prefix:	mc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of cumulative effect of the tax effect of change in accounting principle or new accounting pronouncement.

+ References

No definition available.

+ Details

Name:	mc_NewAccountingPronouncementOrChangeInAccountingPrincipleTaxEffectOfChange
Namespace Prefix:	mc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

- 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

Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

+ Details

Name:	us-gaap_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnEquityOrNetAssets1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Amount after tax of other comprehensive income (loss) attributable to parent entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669625-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569643-111683

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669619-108580

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569616-111683

+ Details

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

- 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

The cumulative amount of the reporting entity's undistributed earnings or deficit.

+ 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(30)(a)(3))
-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)(23)(a)(4))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

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

- 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_AccountingStandardsUpdate201601Member
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_AccountingStandardsUpdate201409Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201609Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=us-gaap_ScenarioAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=mc_DeferredTaxAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AdditionalPaidInCapitalMember
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	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2017
Accounting Policies [Line Items]
Account receivables	$ 19,879	$ 26,738
Interest Income	990	653	$ 408
Retained earnings (accumulated deficit)	(324,192)	(237,782)
Accumulated other comprehensive income	1,452	(453)	1,827
Assets and Liabilities, Lessee
Operating Lease, Liability	197,625
Operating lease, right-of-use asset	$ 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
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 2016-01
Accounting Policies [Line Items]
Retained earnings (accumulated deficit)			(317)
Accumulated other comprehensive income			317
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 2014-09
Revenue and Expense Recognition and Equity-Based Compensation
Cumulative effect on retained earnings and APIC			3,722
Tax effect of adjustment			567
Net change resulting from cumulative effect on retained earnings and APIC			$ 3,155
ASU 2016-09 \| Adjusted \| Deferred Tax Assets
Revenue and Expense Recognition and Equity-Based Compensation
Tax effect of adjustment					$ 658
ASU 2016-09 \| Adjusted \| Retained Earnings (Accumulated Deficit)
Revenue and Expense Recognition and Equity-Based Compensation
Cumulative effect on retained earnings and APIC					(4,855)
Tax effect of adjustment					658
ASU 2016-09 \| Adjusted \| Additional Paid-in Capital
Revenue and Expense Recognition and Equity-Based Compensation
Cumulative effect on retained earnings and APIC					$ 4,855
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