Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenue	$ 19,820	$ 17,224	$ 16,652
Operating Income (Loss)	4,312	3,066	2,677
Total Assets	34,388	33,049	31,357
Depreciation and Amortization	747	741	647
Capital Expenditures	406	348	421
Equity method income			12
Equity method investments	207	280
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,085	10,337	9,599
Interest income, fiduciary assets	15	46	105
Consulting
Segment Reporting Information [Line Items]
Revenue	7,789	6,976	7,143
Operating Segments
Segment Reporting Information [Line Items]
Revenue	19,874	17,313	16,742
Operating Income (Loss)	4,584	3,340	3,043
Total Assets	32,342	30,183	35,820
Depreciation and Amortization	676	674	572
Capital Expenditures	323	277	334
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	12,085	10,337	9,599
Operating Income (Loss)	3,080	2,346	1,833
Total Assets	21,996	20,612	26,098
Depreciation and Amortization	505	500	416
Capital Expenditures	214	170	184
Interest income, fiduciary assets	15	46	105
Equity method income	31	27	25
Gain (loss) on consolidation of business	267
Pre-tax gain on sale	50
Equity method investments	53	165	179
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	7,789	6,976	7,143
Operating Income (Loss)	1,504	994	1,210
Total Assets	10,346	9,571	9,722
Depreciation and Amortization	171	174	156
Capital Expenditures	109	107	150
Equity method income		5	16
Equity method investments	5	5	149
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(54)	(89)	(90)
Operating Income (Loss)	(272)	(274)	(366)
Total Assets	2,046	2,866	(4,463)
Depreciation and Amortization	71	67	75
Capital Expenditures	83	71	87
Inter-segment | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	5	5	8
Inter-segment | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 49	$ 84	$ 82