Segment Information (Details For MMC's Operating Segments) (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
Operating Income (Loss)	4,280	4,312	3,066
Total Assets	33,454	34,388	33,049
Depreciation and Amortization	719	747	741
Capital Expenditures	470	406	348
Equity method investments	215	207
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,645	12,085	10,337
Interest income, fiduciary assets	120	15	46
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
Operating Income (Loss)	4,642	4,584	3,340
Total Assets	32,808	32,342	30,183
Depreciation and Amortization	627	676	674
Capital Expenditures	392	323	277
Pre-tax gain on sale		(50)
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,645	12,085	10,337
Operating Income (Loss)	3,089	3,080	2,346
Total Assets	22,776	21,996	20,612
Depreciation and Amortization	469	505	500
Capital Expenditures	283	214	170
Interest income, fiduciary assets	120	15	46
Equity method income	12	31	27
Gain (loss) on consolidation of business	(27)
Pre-tax gain on sale		(50)
Equity method investments	50	53	165
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	8,139	7,789	6,976
Operating Income (Loss)	1,553	1,504	994
Total Assets	10,032	10,346	9,571
Depreciation and Amortization	158	171	174
Capital Expenditures	109	109	107
Equity method income	1		5
Equity method investments	6	5	5
Operating Segments | Marsh India
Segment Reporting Information [Line Items]
Pre-tax gain on sale		(267)
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(64)	(54)	(89)
Operating Income (Loss)	(362)	(272)	(274)
Total Assets	646	2,046	2,866
Depreciation and Amortization	92	71	67
Capital Expenditures	78	83	71
Inter-segment | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	5	5	5
Inter-segment | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 59	$ 49	$ 84