Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 7,061	$ 6,473
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	7,076	6,487
Risk and Insurance Services | Operating Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income	103	122
Revenue	4,762	4,273
Risk and Insurance Services | Subtotal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	4,659	4,151
Risk and Insurance Services | Total Marsh | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,453	3,003
Risk and Insurance Services | Guy Carpenter | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,206	1,148
Risk and Insurance Services | Total International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,518	1,486
Risk and Insurance Services | EMEA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,059	1,025
Risk and Insurance Services | Asia Pacific | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	335	336
Risk and Insurance Services | Latin America | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	124	125
Risk and Insurance Services | U.S./Canada | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,935	1,517
Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,314	2,214
Consulting | Mercer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,496	1,425
Consulting | Mercer | Wealth | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	670	672
Consulting | Mercer | Health | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	608	538
Consulting | Mercer | Career | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	218	215
Consulting | Oliver Wyman Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 818	$ 789