XML 58 R45.htm IDEA: XBRL DOCUMENT
v3.24.0.1

Summary of Significant Accounting Policies (Narrative) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2023 USD ($) segment | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Number of business segments (segment) | segment | 2 |  |
Restricted cash | $ 486,000,000 | $ 348,000,000 |
Investment income | 5,000,000 | 21,000,000 | $ 61,000,000
Indefinite lived identified intangible assets | 0 | 0 |
Capitalized computer software costs | 519,000,000 | 492,000,000 |
Accumulated amortization | 2,000,000,000 | 1,800,000,000 |
Liability for errors and omission, current and noncurrent | 385,000,000 | 419,000,000 |
Net uncollected premiums and claims and related payables | 13,800,000,000 | 13,000,000,000 |
Total assets | 48,030,000,000 | 44,114,000,000 | 44,010,000,000
Risk and Insurance Services |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Interest income, fiduciary assets | 453,000,000 | 120,000,000 | $ 15,000,000
Accounts Payable and Accrued Liabilities |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Liabilities for errors and omissions, current | $ 71,000,000 | $ 64,000,000 |
Minimum | Customer Lists |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Intangible assets, useful life | 10 years |  |
Maximum | Customer Lists |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Intangible assets, useful life | 15 years |  |
Furniture and equipment | Minimum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 3 years |  |
Furniture and equipment | Maximum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 10 years |  |
Building | Minimum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 30 years |  |
Building | Maximum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 40 years |  |
Office Space | Lease Obligations | Lease Concentration Risk |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Concentration risk (as a percent) | 98.00% |  |
Software Development | Minimum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 3 years |  |
Software Development | Maximum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 10 years |  |

X
- Definition Interest income earned on both U.S. and foreign fiduciary funds held in trust. + References No definition available. + Details
Name: | mmc_InterestIncomeFiduciaryAssets
Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Liabilities for Errors and Omissions, Current + References No definition available. + Details
Name: | mmc_LiabilitiesForErrorsAndOmissionsCurrent
Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Liability for Errors and Omissions, Current and Noncurrent + References No definition available. + Details
Name: | mmc_LiabilityForErrorsAndOmissionsCurrentAndNoncurrent
Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Net uncollected premiums and claims and the related payables are not assets and liabilities of MMC and are not included in the accompanying consolidated balance sheets. + References No definition available. + Details
Name: | mmc_UncollectedPremiumsandClaimsNetofRelatedPayables
Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 810 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (bb) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481203/810-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 810 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 25 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481231/810-10-45-25 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(g)(1)(ii)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 323 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481687/323-10-50-3 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 28 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482907/825-10-50-28 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 852 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481372/852-10-55-10 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480167/946-830-55-12 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(12)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 22 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482810/280-10-50-22 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(8)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1 Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(18)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1A -Subparagraph (SX 210.13-01(a)(4)(i)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1A Reference 13: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1A -Subparagraph (SX 210.13-01(a)(4)(ii)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1A Reference 14: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1A -Subparagraph (SX 210.13-01(a)(4)(iii)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1A Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1A -Subparagraph (SX 210.13-01(a)(4)(iii)(A)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1A Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1A -Subparagraph (SX 210.13-01(a)(4)(iv)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1A Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1A -Subparagraph (SX 210.13-01(a)(5)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1A Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1B -Subparagraph (SX 210.13-02(a)(4)(i)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1B Reference 19: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1B -Subparagraph (SX 210.13-02(a)(4)(iii)(A)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1B Reference 20: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1B -Subparagraph (SX 210.13-02(a)(4)(iii)(B)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1B Reference 21: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1B -Subparagraph (SX 210.13-02(a)(4)(iv)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1B Reference 22: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1B -Subparagraph (SX 210.13-02(a)(5)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480097/470-10-S99-1B Reference 23: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 852 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481404/852-10-50-7 Reference 24: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 30 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482810/280-10-50-30 Reference 25: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 32 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482810/280-10-50-32 Reference 26: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(11)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 + Details
Name: | us-gaap_Assets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition For each balance sheet presented, the amount of accumulated amortization for capitalized computer software costs. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 985 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481283/985-20-50-1 + Details
Name: | us-gaap_CapitalizedComputerSoftwareAccumulatedAmortization
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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 -Topic 985 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481283/985-20-50-1 + Details
Name: | us-gaap_CapitalizedComputerSoftwareNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 42 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482810/280-10-50-42 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482907/825-10-50-21 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482907/825-10-50-20 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482861/275-10-50-18 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482861/275-10-50-20 + Details
Name: | us-gaap_ConcentrationRiskPercentage1
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of realized and unrealized gain (loss) on investment. + References Reference 1: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2 + Details
Name: | us-gaap_GainLossOnInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 350 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482665/350-30-50-2 + Details
Name: | us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 405 -SubTopic 50 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147477123/405-50-65-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 13 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482477/820-10-65-13 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 13 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482477/820-10-65-13 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-5 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-5 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-5 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-5 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 14: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1)(iii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (h)(1)(iv) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (i)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (i)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480528/815-20-65-6 Reference 19: http://www.xbrl.org/2003/role/disclosureRef -Topic 832 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483482/832-10-65-1 Reference 20: http://www.xbrl.org/2003/role/disclosureRef -Topic 832 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483482/832-10-65-1 Reference 21: http://www.xbrl.org/2003/role/disclosureRef -Topic 805 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479845/805-20-65-3 Reference 22: http://www.xbrl.org/2003/role/disclosureRef -Topic 805 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479845/805-20-65-3 Reference 23: http://www.xbrl.org/2003/role/disclosureRef -Topic 805 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 3 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479845/805-20-65-3 Reference 24: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479832/842-10-65-5 Reference 25: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479832/842-10-65-5 Reference 26: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479832/842-10-65-5 Reference 27: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 5 -Subparagraph (d)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479832/842-10-65-5 Reference 28: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-2 Reference 29: http://www.xbrl.org/2003/role/disclosureRef -Topic 848 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483550/848-10-65-2 Reference 30: http://www.xbrl.org/2003/role/disclosureRef -Topic 848 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483550/848-10-65-2 Reference 31: http://www.xbrl.org/2003/role/disclosureRef -Topic 848 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483550/848-10-65-2 Reference 32: http://www.xbrl.org/2003/role/disclosureRef -Topic 848 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a)(3)(iii)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483550/848-10-65-2 Reference 33: http://www.xbrl.org/2003/role/disclosureRef -Topic 848 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a)(3)(iii)(03) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483550/848-10-65-2 Reference 34: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 35: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 36: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 37: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 38: http://www.xbrl.org/2003/role/disclosureRef -Topic 105 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479343/105-10-65-6 Reference 39: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 40: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 41: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 42: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (f)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 43: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (f)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 44: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 45: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(2)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 46: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(2)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 47: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(2)(iii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 48: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(2)(iv) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 49: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (h)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 50: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (h)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2 Reference 51: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 5 -Subparagraph (SAB Topic 11.M.Q2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480530/250-10-S99-5 Reference 52: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 53: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 54: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 55: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 8 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482615/740-10-65-8 Reference 56: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 8 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482615/740-10-65-8 Reference 57: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 8 -Subparagraph (d)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482615/740-10-65-8 Reference 58: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 8 -Subparagraph (d)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482615/740-10-65-8 Reference 59: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 60: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (e)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 61: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 4 -Subparagraph (e)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479654/326-10-65-4 Reference 62: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 15 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480336/718-10-65-15 Reference 63: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 15 -Subparagraph (f)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480336/718-10-65-15 Reference 64: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 15 -Subparagraph (f)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480336/718-10-65-15 Reference 65: http://www.xbrl.org/2003/role/disclosureRef -Topic 926 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483194/926-20-65-2 Reference 66: http://www.xbrl.org/2003/role/disclosureRef -Topic 926 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483194/926-20-65-2 Reference 67: http://www.xbrl.org/2003/role/disclosureRef -Topic 926 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483194/926-20-65-2 Reference 68: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-3 Reference 69: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-1 Reference 70: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-1 Reference 71: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-1 Reference 72: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483443/250-10-50-1 Reference 73: http://www.xbrl.org/2003/role/disclosureRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482635/260-10-55-15 Reference 74: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483421/250-10-45-6 Reference 75: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482833/825-10-65-6 Reference 76: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482833/825-10-65-6 Reference 77: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482833/825-10-65-6 Reference 78: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 6 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482833/825-10-65-6 Reference 79: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 80: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 81: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 82: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 83: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 84: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 85: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 86: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (e)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 87: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (e)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 88: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (e)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 89: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480175/815-40-65-1 Reference 90: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481925/310-20-65-2 Reference 91: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 20 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481925/310-20-65-2 Reference 92: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480424/946-10-50-3 + Details
Name: | us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482810/280-10-50-18 + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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: | instant

X
- Definition Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482913/230-10-50-8 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-4 + Details
Name: | us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mmc_RiskandInsuranceServicesSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_AccountsPayableAndAccruedLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerListsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=srt_OfficeBuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByBenchmarkAxis=mmc_LeaseObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByTypeAxis=mmc_LeaseConcentrationRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_SoftwareDevelopmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |