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Segment Information (Geographic Area) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		$ 24,458	$ 22,736	$ 20,720
Fixed assets, net		859	882	871
Risk and Insurance Services \| Guy Carpenter Reinsurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Gain from the settlement	$ 58		58
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		24,528	22,798	20,784
Operating Segments \| Consulting Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		9,133	8,709	8,139
Operating Segments \| Consulting Segment \| Mercer U.S. Affinity Business
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale		(35)		112
Operating Segments \| Consulting Segment \| Mercer U K Pension Administration Businesses
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale				70
Operating Segments \| Risk and Insurance Services
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		15,395	14,089	12,645
Operating Segments \| Risk and Insurance Services \| Marsh Insurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		12,851	11,657	10,585
Operating Segments \| Risk and Insurance Services \| Guy Carpenter Reinsurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		2,544	2,432	2,060
Corporate/Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		(70)	(62)	(64)
United States
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		11,671	10,924	10,215
Fixed assets, net		494	468	473
United Kingdom
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		3,595	3,555	3,114
Fixed assets, net		150	168	166
Other
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		9,262	8,319	7,455
Fixed assets, net		$ 215	$ 246	$ 232