Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,143	$ 3,090
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,161	1,116
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	226	237
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	452	484
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	1,009	979
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	295	274
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,118	1,184
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	788	742
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	420	424
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	486	451
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	336	294
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	$ (5)	$ (5)