Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,696	$ 2,702	$ 9,349	$ 9,113
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,338	1,329	4,094	3,900
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	250	230	814	748
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	331	340	1,348	1,368
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	22	22	76	94
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	403	438	1,930	2,028
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	352	343	1,087	975
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(2)	(4)	(13)	(15)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,482	1,505	4,893	4,788
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	396	353	1,909	1,775
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	494	497	1,546	1,503
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 326	$ 351	$ 1,014	$ 1,062