XML 66 R44.htm IDEA: XBRL DOCUMENT
v3.21.1

Revenue - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Disaggregation of Revenue [Line Items] |  |
Revenue recognized that was included in the contract liability balance at the beginning of the period | $ 238 | $ 289
Performance obligation satisfied in previous period | 34 | $ 29
Marsh Insurance Group |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, remaining performance obligation | 40 |
Mercer Consulting Group |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, remaining performance obligation | 169 |
Oliver Wyman Group Consulting Group |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, remaining performance obligation | $ 2 |

X
- Definition Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045 + Details
Name: | us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of revenue recognized from performance obligation satisfied or partially satisfied in previous reporting periods. Includes, but is not limited to, change in transaction price. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 12A -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL108322424-203045 + Details
Name: | us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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 No definition available. + Details
Name: | us-gaap_DisaggregationOfRevenueLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of transaction price allocated to performance obligation that has not been recognized as revenue. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130556-203045 + Details
Name: | us-gaap_RevenueRemainingPerformanceObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_SubsegmentsAxis=mmc_MarshInsuranceGroupMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=mmc_MercerConsultingGroupMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsegmentsAxis=mmc_OliverWymanGroupConsultingGroupMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |