Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,349	$ 2,340	$ 8,000	$ 7,089
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,215		3,456
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	203		683
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	264		1,161
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	399		1,871
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	268		829
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,029		3,379
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	279		1,401
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	501		1,356
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	278		1,038
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	263		834
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	$ (1)		$ (8)