Revenue - Disaggregation of 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
Disaggregation of Revenue [Line Items]
Revenue			$ 22,736	$ 20,720	$ 19,820
Loss on consolidation of entity		$ 39	0	39	0
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			22,798	20,784	19,874
Mercer U.S. Affinity Business | Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale					50
Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Fiduciary interest income			453	120	15
Risk and Insurance Services | Operating Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income			453	120	15
Revenue			14,089	12,645	12,085
Pre-tax gain (loss) on sale					50
Risk and Insurance Services | Subtotal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			13,636	12,525	12,070
Risk and Insurance Services | Marsh | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			11,378	10,505	10,203
Revenue			11,657	10,585	10,214
Risk and Insurance Services | Guy Carpenter
Disaggregation of Revenue [Line Items]
Gain from the settlement	$ 58		58
Risk and Insurance Services | Guy Carpenter | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			2,258	2,020	1,867
Revenue			2,432	2,060	1,871
Risk and Insurance Services | Marsh India | Operating Segments
Disaggregation of Revenue [Line Items]
Gain (loss) on consolidation of business					267
Risk and Insurance Services | Total International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			5,116	4,714	4,861
Risk and Insurance Services | EMEA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			3,262	2,997	3,236
Risk and Insurance Services | Asia Pacific | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			1,295	1,215	1,172
Risk and Insurance Services | Latin America | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			559	502	453
Risk and Insurance Services | U.S./Canada | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income			6,262	5,791	5,342
Risk and Insurance Services | RUSSIAN FEDERATION | Operating Segments
Disaggregation of Revenue [Line Items]
Loss on consolidation of entity				27
Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			8,709	8,139	7,789
Consulting | Wealth | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			2,507	2,366	2,509
Consulting | Health | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			2,061	2,017	1,855
Consulting | Career | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			1,019	962	890
Consulting | Mercer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			5,587	5,345	5,254
Consulting | Oliver Wyman Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			3,122	2,794	$ 2,535
Consulting | Mercer U.S. Affinity Business | Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale				112
Consulting | Small Individual Financial Advisory Business | Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax gain (loss) on sale			$ (17)
Consulting | RUSSIAN FEDERATION | Operating Segments
Disaggregation of Revenue [Line Items]
Loss on consolidation of entity				$ 12