Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,670	$ 3,525
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,417	1,308
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	276	250
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	528	530
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	29	33
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	1,058	1,091
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	362	313
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(8)	(7)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,719	1,640
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	976	922
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	638	615
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 345	$ 355