Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,702	$ 2,385	$ 9,113	$ 8,101
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,329	1,169	3,900	3,503
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	230	200	748	647
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	340	332	1,368	1,276
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	22	19	94	63
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	438	372	2,028	1,731
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	343	293	975	881
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(4)	(6)	(15)	(13)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,505	1,320	4,788	4,197
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	353	321	1,775	1,617
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	497	423	1,503	1,316
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 351	$ 327	$ 1,062	$ 984