Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 5,924	$ 5,549
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	5,937	5,559
Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	3,906	3,549
Fiduciary interest income	91	4
Risk and Insurance Services \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	3,906	3,549
Fiduciary interest income	91	4
Gain (loss) on consolidation of business	(27)
Risk and Insurance Services \| Total International
Disaggregation of Revenue [Line Items]
Revenue	1,359	1,267
Risk and Insurance Services \| EMEA
Disaggregation of Revenue [Line Items]
Revenue	932	869
Risk and Insurance Services \| Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	312	294
Risk and Insurance Services \| Latin America
Disaggregation of Revenue [Line Items]
Revenue	115	104
Risk and Insurance Services \| U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,385	1,279
Consulting
Disaggregation of Revenue [Line Items]
Revenue	2,031	2,010
Consulting \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,031	2,010
Consulting \| Wealth
Disaggregation of Revenue [Line Items]
Revenue	581	617
Consulting \| Health
Disaggregation of Revenue [Line Items]
Revenue	545	524
Consulting \| Career
Disaggregation of Revenue [Line Items]
Revenue	218	202
Oliver Wyman Group \| Operating Segments
Disaggregation of Revenue [Line Items]
Gain (loss) on consolidation of business	(12)
Small Individual Financial Advisory Business \| Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax loss on sale	19
Marsh & Guy Carpenter \| Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	3,815	3,545
Marsh \| Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	2,744	2,546
Marsh \| Risk and Insurance Services \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,800	2,549
Guy Carpenter \| Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	1,071	999
Guy Carpenter \| Risk and Insurance Services \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,106	1,000
Mercer \| Consulting
Disaggregation of Revenue [Line Items]
Revenue	1,344	1,343
Mercer \| Consulting \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,344	1,343
Oliver Wyman Group \| Consulting
Disaggregation of Revenue [Line Items]
Revenue	687	667
Oliver Wyman Group \| Consulting \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 687	$ 667

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Interest income earned on both U.S. and foreign fiduciary funds held in trust.

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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.

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Amount of gain (loss) from sale and disposal of integrated set of activities and assets capable of being conducted and managed for purpose of providing return in form of dividend, lower cost, or other economic benefit to investor, owner, member and participant.

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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.

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