Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified
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3 Months Ended |
12 Months Ended |
Dec. 31, 2014
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Sep. 30, 2014
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Jun. 30, 2014
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Mar. 31, 2014
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Dec. 31, 2013
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Sep. 30, 2013
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Jun. 30, 2013
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Mar. 31, 2013
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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Segment Reporting Information [Line Items] |
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Revenue |
$ 3,246us-gaap_SalesRevenueServicesNet |
$ 3,141us-gaap_SalesRevenueServicesNet |
$ 3,300us-gaap_SalesRevenueServicesNet |
$ 3,264us-gaap_SalesRevenueServicesNet |
$ 3,115us-gaap_SalesRevenueServicesNet |
$ 2,932us-gaap_SalesRevenueServicesNet |
$ 3,088us-gaap_SalesRevenueServicesNet |
$ 3,126us-gaap_SalesRevenueServicesNet |
$ 12,951us-gaap_SalesRevenueServicesNet |
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$ 12,261us-gaap_SalesRevenueServicesNet |
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$ 11,924us-gaap_SalesRevenueServicesNet |
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Fixed Assets, Net |
809us-gaap_PropertyPlantAndEquipmentNet |
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828us-gaap_PropertyPlantAndEquipmentNet |
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809us-gaap_PropertyPlantAndEquipmentNet |
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828us-gaap_PropertyPlantAndEquipmentNet |
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809us-gaap_PropertyPlantAndEquipmentNet |
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United States [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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5,865us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = country_US |
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5,485us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = country_US |
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5,300us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = country_US |
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Fixed Assets, Net |
483us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_US |
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494us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_US |
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483us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_US |
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494us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_US |
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494us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_US |
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United Kingdom [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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2,111us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = country_GB |
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1,979us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = country_GB |
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1,960us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = country_GB |
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Fixed Assets, Net |
120us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_GB |
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121us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_GB |
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120us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_GB |
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121us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_GB |
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121us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = country_GB |
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Europe [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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2,077us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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1,943us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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1,879us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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Fixed Assets, Net |
60us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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64us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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60us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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64us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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63us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |
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Asia Pacific [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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1,420us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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1,396us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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1,346us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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Fixed Assets, Net |
62us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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72us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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62us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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72us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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62us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = us-gaap_AsiaPacificMember |
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Other Geographic Areas [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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1,517us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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1,494us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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1,478us-gaap_SalesRevenueServicesNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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Fixed Assets, Net |
84us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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77us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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84us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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77us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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69us-gaap_PropertyPlantAndEquipmentNet / us-gaap_StatementGeographicalAxis = mmc_OtherGeographicAreasMember |
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Operating Segments [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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12,990us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember |
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12,297us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember |
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11,963us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember |
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Operating Segments [Member] | Risk and Insurance Services Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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6,931us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember |
[1] |
6,596us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember |
[1] |
6,350us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember |
[1] |
Operating Segments [Member] | Consulting Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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6,059us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember |
[2] |
5,701us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember |
[2] |
5,613us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember |
[2] |
Intersegment Eliminations [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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(39)us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember |
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(36)us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember |
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(39)us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember |
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Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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4us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember |
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5us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember |
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5us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember |
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Intersegment Eliminations [Member] | Consulting Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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35us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember |
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31us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember |
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34us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_IntersegmentEliminationMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember |
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Marsh Insurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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5,774us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember / us-gaap_SubsegmentsAxis = mmc_MarshInsuranceGroupMember |
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5,461us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember / us-gaap_SubsegmentsAxis = mmc_MarshInsuranceGroupMember |
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5,265us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember / us-gaap_SubsegmentsAxis = mmc_MarshInsuranceGroupMember |
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Guy Carpenter Reinsurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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1,157us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember / us-gaap_SubsegmentsAxis = mmc_GuyCarpenterReinsuranceGroupMember |
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1,135us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember / us-gaap_SubsegmentsAxis = mmc_GuyCarpenterReinsuranceGroupMember |
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1,085us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_RiskandInsuranceServicesSegmentMember / us-gaap_SubsegmentsAxis = mmc_GuyCarpenterReinsuranceGroupMember |
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Mercer Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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4,350us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember / us-gaap_SubsegmentsAxis = mmc_MercerConsultingGroupMember |
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4,241us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember / us-gaap_SubsegmentsAxis = mmc_MercerConsultingGroupMember |
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4,147us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember / us-gaap_SubsegmentsAxis = mmc_MercerConsultingGroupMember |
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Oliver Wyman Group Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member] |
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Segment Reporting Information [Line Items] |
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Revenue |
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$ 1,709us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember / us-gaap_SubsegmentsAxis = mmc_OliverWymanGroupConsultingGroupMember |
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$ 1,460us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember / us-gaap_SubsegmentsAxis = mmc_OliverWymanGroupConsultingGroupMember |
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$ 1,466us-gaap_SalesRevenueServicesNet / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = mmc_ConsultingSegmentMember / us-gaap_SubsegmentsAxis = mmc_OliverWymanGroupConsultingGroupMember |
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