Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenue	$ 12,193	$ 11,066	$ 11,013
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	5,459	5,032	5,016
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,027	911	919
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,681	1,579	1,502
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	127	84	75
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,565	2,236	2,263
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,334	1,224	1,238
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	(19)	(17)	(14)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	6,635	5,861	5,857
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,997	1,814	1,686
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	2,154	2,067	2,104
Wealth Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,426	$ 1,341	$ 1,380