Segment Information (Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenue		$ 22,736	$ 20,720	$ 19,820
Operating Income (Loss)		5,282	4,280	4,312
Total Assets		48,030	44,114	44,010
Depreciation and Amortization		713	719	747
Capital Expenditures		416	470	406
Equity method investments		266	215
Deconsolidation, Gain (Loss), Amount	$ (39)	0	(39)	0
Risk and Insurance Services
Segment Reporting Information [Line Items]
Interest income, fiduciary assets		453	120	15
Operating Segments
Segment Reporting Information [Line Items]
Revenue		22,798	20,784	19,874
Operating Income (Loss)		5,611	4,642	4,584
Total Assets		45,910	43,468	41,964
Depreciation and Amortization		639	627	676
Capital Expenditures		347	392	323
Operating Segments | Mercer U.S. Affinity Business
Segment Reporting Information [Line Items]
Pre-tax gain (loss) on sale				50
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue		14,089	12,645	12,085
Operating Income (Loss)		3,945	3,089	3,080
Total Assets		34,449	33,022	31,233
Depreciation and Amortization		487	469	505
Capital Expenditures		242	283	214
Interest income, fiduciary assets		453	120	15
Equity method income		18	12	31
Pre-tax gain (loss) on sale				50
Equity method investments		57	50	53
Operating Segments | Risk and Insurance Services | RUSSIAN FEDERATION
Segment Reporting Information [Line Items]
Deconsolidation, Gain (Loss), Amount			(27)
Operating Segments | Risk and Insurance Services | Marsh India
Segment Reporting Information [Line Items]
Gain (loss) on consolidation of business				267
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue		8,709	8,139	7,789
Operating Income (Loss)		1,666	1,553	1,504
Total Assets		11,461	10,446	10,731
Depreciation and Amortization		152	158	171
Capital Expenditures		105	109	109
Equity method income			1
Equity method investments		6	6	5
Operating Segments | Consulting | RUSSIAN FEDERATION
Segment Reporting Information [Line Items]
Deconsolidation, Gain (Loss), Amount			(12)
Operating Segments | Consulting | Small Individual Financial Advisory Business
Segment Reporting Information [Line Items]
Pre-tax gain (loss) on sale		(17)
Operating Segments | Consulting | Mercer U.S. Affinity Business
Segment Reporting Information [Line Items]
Pre-tax gain (loss) on sale			112
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue		(62)	(64)	(54)
Operating Income (Loss)		(329)	(362)	(272)
Total Assets		2,120	646	2,046
Depreciation and Amortization		74	92	71
Capital Expenditures		69	78	83
Inter-segment | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue		6	5	5
Inter-segment | Consulting
Segment Reporting Information [Line Items]
Revenue		$ 56	$ 59	$ 49