Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Revenue	$ 20,720	$ 19,820	$ 17,224
Fixed Assets, Net	871	847	856
United States
Segment Reporting Information [Line Items]
Revenue	10,215	9,343	8,168
Fixed Assets, Net	473	484	492
United Kingdom
Segment Reporting Information [Line Items]
Revenue	3,114	3,130	2,818
Fixed Assets, Net	166	116	115
Continental Europe
Segment Reporting Information [Line Items]
Revenue	3,223	3,219	2,881
Fixed Assets, Net	68	68	74
Asia Pacific
Segment Reporting Information [Line Items]
Revenue	2,537	2,617	2,093
Fixed Assets, Net	81	96	105
Other
Segment Reporting Information [Line Items]
Revenue	1,695	1,565	1,353
Fixed Assets, Net	83	83	70
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,645	12,085	10,337
Risk and Insurance Services \| Asia Pacific
Segment Reporting Information [Line Items]
Revenue	1,333	1,462	1,059
Consulting
Segment Reporting Information [Line Items]
Revenue	8,139	7,789	6,976
Operating Segments
Segment Reporting Information [Line Items]
Revenue	20,784	19,874	17,313
Pre-tax gain on sale		(50)
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,645	12,085	10,337
Pre-tax gain on sale		(50)
Gain (loss) on consolidation of business	27
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	8,139	7,789	6,976
Operating Segments \| Marsh India
Segment Reporting Information [Line Items]
Pre-tax gain on sale		(267)
Operating Segments \| Mercer U.S. Affinity Business
Segment Reporting Information [Line Items]
Pre-tax gain on sale	(112)
Operating Segments \| Oliver Wyman Group
Segment Reporting Information [Line Items]
Gain (loss) on consolidation of business	12
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(64)	(54)	(89)
Marsh \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	10,505	10,203	8,595
Marsh \| Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	10,585	10,214	8,628
Guy Carpenter \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	2,020	1,867	1,696
Guy Carpenter \| Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	2,060	1,871	1,709
Mercer \| Consulting
Segment Reporting Information [Line Items]
Revenue	5,345	5,254	4,928
Mercer \| Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	5,345	5,254	4,928
Oliver Wyman Group \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,794	2,535	2,048
Oliver Wyman Group \| Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	$ 2,794	$ 2,535	$ 2,048

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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 after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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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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