Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,983	$ 2,886	$ 6,653	$ 6,411
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,339	1,263	2,756	2,571
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	288	268	564	518
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	489	498	1,017	1,028
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	25	39	54	72
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	469	499	1,527	1,590
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	373	319	735	632
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(3)	(4)	(11)	(11)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,692	1,643	3,411	3,283
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	537	500	1,513	1,422
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	414	391	1,052	1,006
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 343	$ 356	$ 688	$ 711