Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,871	$ 3,670
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,495	1,417
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	301	276
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	554	528
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	30	29
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	1,102	1,058
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	389	362
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(5)	(8)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,778	1,719
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,077	976
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	671	638
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 350	$ 345