Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,953	$ 2,696	$ 10,001	$ 9,349
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,400	1,338	4,322	4,094
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	281	250	890	814
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	396	331	1,456	1,348
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	24	22	82	76
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	479	403	2,093	1,930
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	373	352	1,158	1,087
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(1)	(2)	(9)	(13)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,585	1,482	5,137	4,893
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	465	396	2,149	1,909
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	552	494	1,670	1,546
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 352	$ 326	$ 1,054	$ 1,014