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v3.21.1

Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Fiduciary Assets and Liabilities [Line Items] |  |  |
Net uncollected premiums and claims receivable and payable | $ 12,500 |  | $ 11,200
Risk and Insurance Services Segment |  |  |
Fiduciary Assets and Liabilities [Line Items] |  |  |
Interest on fiduciary funds | 5 | $ 23 |
Operating Segments | Risk and Insurance Services Segment |  |  |
Fiduciary Assets and Liabilities [Line Items] |  |  |
Interest on fiduciary funds | $ 5 | $ 23 |

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- Definition Fiduciary Assets and Liabilities [Line Items] + References No definition available. + Details
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Namespace Prefix: | mmc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Interest income earned on both U.S. and foreign fiduciary funds held in trust. + References No definition available. + Details
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Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Net uncollected premiums and claims and the related payables are not assets and liabilities of MMC and are not included in the accompanying consolidated balance sheets. + References No definition available. + Details
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Namespace Prefix: | mmc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |