Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 12,479	$ 12,193	$ 11,066
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	5,666	5,459	5,032
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,137	1,027	911
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,660	1,681	1,579
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	99	127	84
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,443	2,565	2,236
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,474	1,334	1,224
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	(17)	(19)	(17)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	6,715	6,635	5,861
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	2,190	1,997	1,814
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	2,224	2,154	2,067
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,367	$ 1,426	$ 1,341