Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 6,473	$ 5,924
Total Segments
Disaggregation of Revenue [Line Items]
Revenue	6,487	5,937
Risk and Insurance Services | Total Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income	122	91
Revenue	4,273	3,906
Risk and Insurance Services | Subtotal | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	4,151	3,815
Risk and Insurance Services | Total Marsh | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,003	2,744
Risk and Insurance Services | Guy Carpenter | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,148	1,071
Risk and Insurance Services | Total International | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,486	1,359
Risk and Insurance Services | EMEA | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,025	932
Risk and Insurance Services | Asia Pacific | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	336	312
Risk and Insurance Services | Latin America | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	125	115
Risk and Insurance Services | U.S./Canada | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,517	1,385
Consulting | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	2,214	2,031
Consulting | Mercer | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	1,425	1,344
Consulting | Mercer | Wealth | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	672	581
Consulting | Mercer | Health | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	538	545
Consulting | Mercer | Career | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	215	218
Consulting | Oliver Wyman Group | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 789	$ 687