Leases (Narrative) (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)
Lessee, Lease, Description [Line Items]
ROU assets impairment loss	$ 6	$ 19
Lessee, operating lease, lease not yet commenced, amount	$ 56	$ 56
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract (years)	10 years	10 years
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract (years)	25 years	25 years

Lessee, Operating Lease, Lease Not yet Commenced, Amount

No definition available.

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Namespace Prefix:	mmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147478964/842-20-50-3

+ Details

Name:	us-gaap_LesseeLeaseDescriptionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147478964/842-20-50-3

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of loss from impairment of right-of-use asset from operating lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479365/842-20-25-6

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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- Details

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