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Claims, Lawsuits And Other Contingencies (Details) £ in Millions | Sep. 30, 2018 GBP (£)
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Other Contingencies-Guarantees |
Loss Contingencies [Line Items] |
Amount reinsured by third party (up to) | £ 40

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- Definition Reinsurance Policy, Amount Reinsured By Third Party + References No definition available. + Details
Name: | mmc_ReinsurancePolicyAmountReinsuredByThirdParty
Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_LitigationCaseAxis=mmc_OtherContingenciesGuaranteesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |