Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,760	$ 3,177	$ 7,830	$ 7,048
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,837	1,427	3,349	2,922
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	362	308	685	609
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	548	506	1,131	1,060
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	35	28	68	58
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	566	512	1,775	1,614
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	412	396	822	785
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(15)	(3)	(23)	(8)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	2,015	1,774	3,823	3,552
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	635	607	1,802	1,684
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	662	447	1,395	1,118
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 463	$ 352	$ 833	$ 702