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Leases (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Lessee, Lease, Description [Line Items] |  |
ROU assets impairment loss | $ 8 | $ 1
Lessee, operating lease, lease not yet commenced, amount | $ 59 |
Minimum |  |
Lessee, Lease, Description [Line Items] |  |
Lessee, operating lease, term of contract (years) | 10 years |
Maximum |  |
Lessee, Lease, Description [Line Items] |  |
Lessee, operating lease, term of contract (years) | 25 years |

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- Definition Lessee, Operating Lease, Lease Not yet Commenced, Amount + References No definition available. + Details
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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI https://asc.fasb.org/extlink&oid=128292326&loc=SL77918673-209980 + Details
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Period Type: | instant

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- Definition Amount of loss from impairment of right-of-use asset from operating lease. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 25 -Paragraph 6 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=123399704&loc=SL77918431-209957 + Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration

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- Details
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- Details
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