Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,177	$ 2,983	$ 7,048	$ 6,653
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,427	1,339	2,922	2,756
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	308	288	609	564
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	506	489	1,060	1,017
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	28	25	58	54
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	512	469	1,614	1,527
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	396	373	785	735
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(3)	(3)	(8)	(11)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,774	1,692	3,552	3,411
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	607	537	1,684	1,513
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	447	414	1,118	1,052
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 352	$ 343	$ 702	$ 688