Defined Benefit Plan, Assumptions Used In Calculation, Period Between Negotiations With Plan Trustee

No definition available.

Name:	mmc_DefinedBenefitPlanAssumptionsUsedInCalculationPeriodBetweenNegotiationsWithPlanTrustee
Namespace Prefix:	mmc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Health Care Cost Trend, Entity's Maximum Trend Rate

+ References

No definition available.

+ Details

Name:	mmc_DefinedBenefitPlanHealthCareCostTrendEntitysMaximumTrendRate
Namespace Prefix:	mmc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Market-related Plan Assets Value, Recognition Period of Investment Gains and Losses

+ References

No definition available.

+ Details

Name:	mmc_DefinedBenefitPlanMarketrelatedPlanAssetsValueRecognitionPeriodofInvestmentGainsandLosses
Namespace Prefix:	mmc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Number Of Plans For Which Lump Sum Payments Exceeded Settlement Thresholds

+ References

No definition available.

+ Details

Name:	mmc_DefinedBenefitPlanNumberOfPlansForWhichLumpSumPaymentsExceededSettlementThresholds
Namespace Prefix:	mmc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Number of Shares of Equity Securities Issued By Employer and Related Parties as Percentage Of Plan Assets

+ References

No definition available.

+ Details

Name:	mmc_DefinedBenefitPlanNumberofSharesofEquitySecuritiesIssuedByEmployerandRelatedPartiesasPercentageOfPlanAssets
Namespace Prefix:	mmc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Contribution Plan, Employee Stock Ownership Plan (ESOP), Shares in ESOP, Value

+ References

No definition available.

+ Details

Name:	mmc_DefinedContributionPlanEmployeeStockOwnershipPlanESOPSharesinESOPValue
Namespace Prefix:	mmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Guarantor Obligation, Period

+ References

No definition available.

+ Details

Name:	mmc_GuarantorObligationPeriod
Namespace Prefix:	mmc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(10)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanBenefitObligationPaymentForSettlement
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of contribution expected to be received by defined benefit plan from employer in next fiscal year following current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4587-114921

+ Details

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

- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares of employer stock in which defined benefit plan asset is invested. Includes, but is not limited to, stock issued or managed by related party of employer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (n)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanNumberOfSharesOfEquitySecuritiesIssuedByEmployerAndRelatedPartiesIncludedInPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of plan asset for defined benefit pension plan with projected benefit obligation in excess of plan assets.

+ References

+ Details

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

- Definition

Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

+ References

+ Details

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

- Definition

Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920

+ Details

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

- Definition

Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920

+ Details

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

- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

+ Details

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

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=mmc_PensionandPostretirementBenefitPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_EquityFundsMember
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:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_FixedIncomeFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=us-gaap_ForeignPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=mmc_PensionPlanFundedStatusGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_GB
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanNameAxis=mmc_JardineLloydThompsonUnitedKingdomPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=mmc_UnitedKingdomPensionandPostretirementBenefitPlanDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_GeographicConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Retirement Benefits (Narrative) (Details) $ in Millions		1 Months Ended	12 Months Ended
Retirement Benefits (Narrative) (Details) $ in Millions	Jan. 01, 2017	Nov. 30, 2019 GBP (£)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) plan
Defined Benefit Plan Disclosure [Line Items]
Period to recognize gains or losses on market-related value of assets			5 years
Assumed health care cost trend rate for Medicare eligibles and non-Medicare eligibles			4.93%
Future assumed health care cost trend rate			4.22%
Automatic employer contribution of employee's eligible base pay (as a percent)	4.00%
401(k) Plan's assets invested in common stock			$ 537	$ 556
U.S.
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions, next fiscal year			37
Cost of defined contribution plans			$ 145	139	$ 133
U.S. \| Pension and Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate for Medicare eligibles and non-Medicare eligibles			5.80%
Future assumed health care cost trend rate			4.50%
Cap on employer share of health care trend (as a percent)			5.00%
U.S. \| Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for plans with accumulated benefit obligations in excess of plan assets			$ 6,900	6,300
Aggregate fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets			5,100	4,700
Projected benefit obligation for plans with projected benefit obligations in excess of plan assets			6,900	6,300
Fair value of plan assets for plans with projected benefit obligations in excess of plan assets			$ 5,100	4,700
U.S. \| Post-retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Number of shares held in defined benefit plan (in shares) \| shares			2,000,000
Change in plan assets (as a percent)			4.60%
U.S. \| Equities and equity alternatives
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			64.00%
Actual asset allocation percentage			64.00%
U.S. \| Equities and equity alternatives \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			59.00%
U.S. \| Equities and equity alternatives \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			69.00%
U.S. \| Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			36.00%
Actual asset allocation percentage			36.00%
U.S. \| Fixed income \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			31.00%
U.S. \| Fixed income \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			41.00%
Non-U.S.
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for plans with accumulated benefit obligations in excess of plan assets			$ 3,100	2,700
Aggregate fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets			2,500	2,200
Projected benefit obligation for plans with projected benefit obligations in excess of plan assets			3,300	3,000
Fair value of plan assets for plans with projected benefit obligations in excess of plan assets			2,600	2,300
Cost of defined contribution plans			121	100	$ 80
Non-U.S. \| Pension Plan Funded Status Guarantee
Defined Benefit Plan Disclosure [Line Items]
Annual deficit contributions, period		7 years
Non-U.S. \| Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Non-cash settlement charges			3	$ 7
Expected employer contributions, next fiscal year			87
United Kingdom \| JLT U.K. Plan
Defined Benefit Plan Disclosure [Line Items]
Annual deficit contributions (GBP)			$ 29
United Kingdom \| Pension Plan Funded Status Guarantee
Defined Benefit Plan Disclosure [Line Items]
Annual deficit contributions (GBP) \| £		£ 450,000,000
United Kingdom \| Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Number of plans for Lump Sum payments exceeded settlement thresholds \| plan					2
Non-cash settlement charges					$ 42
Period between legally-prescribed negotiations between the Company and the plans' trustee			3 years
United Kingdom \| Equities and equity alternatives
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			32.00%
Actual asset allocation percentage			33.00%
United Kingdom \| Equities and equity alternatives \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			29.00%
United Kingdom \| Equities and equity alternatives \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			35.00%
United Kingdom \| Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			68.00%
Actual asset allocation percentage			67.00%
United Kingdom \| Fixed income \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			65.00%
United Kingdom \| Fixed income \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage			71.00%
United Kingdom Pension and Postretirement Benefit Plan, Defined Benefit \| United Kingdom \| Geographic Concentration Risk
Defined Benefit Plan Disclosure [Line Items]
Concentration risk (as a percent)			81.00%