Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,997	$ 3,721	$ 12,881	$ 11,551
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	2,084	2,083	6,126	5,432
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	381	263	1,150	948
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	525	437	1,756	1,568
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	42	38	133	106
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	555	492	2,451	2,267
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	410	408	1,265	1,230
Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	2,525	2,355	8,582	7,980
Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,475	1,369	4,311	3,597
Operating Segments | U.S. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,229	1,206	3,774	3,501
Operating Segments | U.S. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	858	880	2,364	1,957
Operating Segments | Americas other than U.S. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	260	195	799	682
Operating Segments | Americas other than U.S. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	121	68	351	266
Operating Segments | U.K. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	303	255	1,144	1,045
Operating Segments | U.K. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	222	182	612	523
Operating Segments | Ireland | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	21	20	67	56
Operating Segments | Ireland | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	21	18	66	50
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	400	361	1,815	1,717
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	155	131	636	550
Operating Segments | Asia Pacific | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	312	318	983	979
Operating Segments | Asia Pacific | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	98	90	282	251
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(3)	(3)	(12)	(26)
Eliminations | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1	1	5	16
Eliminations | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	3	3	7	10
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(3)	(3)	(12)	(26)
Corporate/Eliminations | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	(3)	(3)	(12)	(26)
Corporate/Eliminations | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Corporate/Eliminations | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Corporate/Eliminations | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Corporate/Eliminations | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Corporate/Eliminations | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Commercial Risk Solutions | Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,988	1,852	6,168	5,675
Reinsurance Solutions | Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	537	503	2,414	2,305
Health Solutions | Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	935	870	2,733	2,265
Wealth Solutions | Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	$ 540	$ 499	$ 1,578	$ 1,332