Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Total revenue	$ 13,376	$ 12,479	$ 12,193
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	5,923	5,666	5,459
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,260	1,137	1,027
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,819	1,660	1,681
IRELAND
Disaggregation of Revenue [Line Items]
Total revenue	113	99	127
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,672	2,443	2,565
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,589	1,474	1,334
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	(12)	(17)	(19)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	7,043	6,715	6,635
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	2,481	2,190	1,997
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	2,433	2,224	2,154
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,431	$ 1,367	$ 1,426