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Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Fiduciary Assets and Liabilities [Line Items] |  |  |  |  |
Net uncollected premiums and claims receivable and payable | $ 7,800 |  | $ 7,800 |  | $ 6,800
Risk and Insurance Services Segment |  |  |  |  |
Fiduciary Assets and Liabilities [Line Items] |  |  |  |  |
Interest on fiduciary funds (less than) | 18 |  | 46 |  |
Operating Segments | Risk and Insurance Services Segment |  |  |  |  |
Fiduciary Assets and Liabilities [Line Items] |  |  |  |  |
Interest on fiduciary funds (less than) | 18 | $ 11 | 46 | $ 28 |
Operating Segments | Consulting Segment |  |  |  |  |
Fiduciary Assets and Liabilities [Line Items] |  |  |  |  |
Interest on fiduciary funds (less than) | $ 1 | $ 2 | $ 3 | $ 3 |

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- Definition Fiduciary Assets and Liabilities [Line Items] + References No definition available. + Details
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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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- 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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- Details
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- Details
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- Details
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