Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Total revenue	$ 4,155	$ 3,760	$ 8,884	$ 7,830
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	2,040	1,837	4,042	3,349
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	395	362	769	685
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	610	548	1,231	1,131
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	46	35	91	68
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	634	566	1,896	1,775
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	430	412	855	822
Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	2,866	2,650	6,057	5,625
Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,291	1,125	2,836	2,228
Operating Segments | U.S. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,324	1,217	2,545	2,295
Operating Segments | U.S. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	718	635	1,506	1,077
Operating Segments | Americas other than U.S. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	285	260	539	487
Operating Segments | Americas other than U.S. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	110	102	230	198
Operating Segments | U.K. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	411	379	841	790
Operating Segments | U.K. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	199	169	390	341
Operating Segments | Ireland | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	24	20	46	36
Operating Segments | Ireland | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	22	15	45	32
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	469	428	1,415	1,356
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	165	138	481	419
Operating Segments | Asia Pacific | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	353	346	671	661
Operating Segments | Asia Pacific | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	77	66	184	161
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(2)	(15)	(9)	(23)
Eliminations | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1	10	5	16
Eliminations | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	2	5	4	7
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(2)	(15)	(9)	(23)
Corporate/Eliminations | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	(2)	(15)	(9)	(23)
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	2,178	2,015	4,180	3,823
Reinsurance Solutions | Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	688	635	1,877	1,802
Health Solutions | Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	772	662	1,798	1,395
Wealth Solutions | Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	$ 519	$ 463	$ 1,038	$ 833