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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 4,651 | $ 4,071
Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,911 |
Fiduciary interest income | 23 |
Risk and Insurance Services Segment | EMEA |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 754 |
Risk and Insurance Services Segment | Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 238 |
Risk and Insurance Services Segment | Latin America |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 91 |
Risk and Insurance Services Segment | Total International |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,083 |
Risk and Insurance Services Segment | U.S./Canada |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 978 |
Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,762 |
Consulting Segment | Wealth |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 592 |
Consulting Segment | Health |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 486 |
Consulting Segment | Career |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 173 |
Marsh Insurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,061 |
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 827 |
Marsh & Guy Carpenter | Risk and Insurance Services Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 2,888 |
Mercer Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | 1,251 |
Oliver Wyman Group Consulting Group | Consulting Segment |  |
Disaggregation of Revenue [Line Items] |  |
Revenue | $ 511 |

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- Definition Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045 + Details
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