Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue		$ 24,458	$ 22,736	$ 20,720
U.S. Health And Benefits And U.K Pension Administration Businesses
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale		35
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		24,528	22,798	20,784
Wealth | U.S. Health And Benefits And U.K Pension Administration Businesses
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale		70
Health | U.S. Health And Benefits And U.K Pension Administration Businesses
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale		(35)
Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Fiduciary interest income		497	453	120
Risk and Insurance Services | Operating Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income		497	453	120
Revenue		15,395	14,089	12,645
Risk and Insurance Services | Marsh & Guy Carpenter | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		14,898	13,636	12,525
Risk and Insurance Services | Marsh Insurance Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		12,536	11,378	10,505
Revenue		12,851	11,657	10,585
Risk and Insurance Services | Guy Carpenter Reinsurance Group
Disaggregation of Revenue [Line Items]
Gain from the settlement	$ 58		58
Risk and Insurance Services | Guy Carpenter Reinsurance Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		2,362	2,258	2,020
Revenue		2,544	2,432	2,060
Risk and Insurance Services | Total International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		5,519	5,116	4,714
Risk and Insurance Services | EMEA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		3,530	3,262	2,997
Risk and Insurance Services | Asia Pacific | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		1,414	1,295	1,215
Risk and Insurance Services | Latin America | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		575	559	502
Risk and Insurance Services | U.S./Canada | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income		7,017	6,262	5,791
Consulting Segment | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		9,133	8,709	8,139
Consulting Segment | Operating Segments | Mercer U.S. Affinity Business
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale		(35)		112
Consulting Segment | Wealth | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		2,584	2,507	2,366
Consulting Segment | Health | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		2,100	2,061	2,017
Consulting Segment | Career | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		1,059	1,019	962
Consulting Segment | Mercer Consulting Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		5,743	5,587	5,345
Consulting Segment | Oliver Wyman Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		3,390	3,122	$ 2,794
Pre-tax gain (loss) on sale		20
Consulting Segment | Small Individual Financial Advisory Business | Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale		$ 35	$ 17