Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 11,066	$ 11,013	$ 10,770
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	5,032	5,016	4,677
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	911	919	940
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,579	1,502	1,555
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	84	75	80
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,236	2,263	2,333
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,224	1,238	1,185
Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	4,690	4,673	4,652
Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,814	1,686	1,563
Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,753	1,817	1,865
Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,655	1,667	1,596
Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	1,171	1,184	1,105
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	$ (17)	$ (14)	$ (11)