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Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2023	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenue			$ 22,736	$ 20,720	$ 19,820
Loss on consolidation of entity		$ 39	0	39	0
Fixed assets, net			882	871	847
United States (a)
Segment Reporting Information [Line Items]
Revenue			10,924	10,215	9,343
Fixed assets, net			468	473	484
United Kingdom (b)
Segment Reporting Information [Line Items]
Revenue			3,555	3,114	3,130
Fixed assets, net			168	166	116
Other (c)
Segment Reporting Information [Line Items]
Revenue			8,319	7,455	7,401
Fixed assets, net			246	232	247
Operating Segments
Segment Reporting Information [Line Items]
Revenue			22,798	20,784	19,874
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue			14,089	12,645	12,085
Pre-tax gain (loss) on sale					50
Operating Segments \| Risk and Insurance Services \| RUSSIAN FEDERATION
Segment Reporting Information [Line Items]
Loss on consolidation of entity				27
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue			8,709	8,139	7,789
Operating Segments \| Consulting \| RUSSIAN FEDERATION
Segment Reporting Information [Line Items]
Loss on consolidation of entity				12
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue			(62)	(64)	(54)
Marsh \| Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue			11,657	10,585	10,214
Guy Carpenter \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Gain from the settlement	$ (58)		(58)
Guy Carpenter \| Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue			2,432	2,060	1,871
Mercer \| Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue			5,587	5,345	5,254
Oliver Wyman Group \| Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue			3,122	2,794	2,535
Small Individual Financial Advisory Business \| Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Pre-tax gain (loss) on sale			$ (17)
Mercer U.S. Affinity Business \| Operating Segments
Segment Reporting Information [Line Items]
Pre-tax gain (loss) on sale					$ 50
Mercer U.S. Affinity Business \| Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Pre-tax gain (loss) on sale				$ 112