Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Total revenue	$ 4,070	$ 3,871
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,512	1,495
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	323	301
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	583	554
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	33	30
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	1,209	1,102
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	410	389
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(8)	(5)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,808	1,778
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,167	1,077
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	733	671
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 370	$ 350