Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,561	$ 2,368	$ 5,651	$ 4,749
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,125		2,241
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	243		480
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	413		897
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	493		1,472
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	287		561
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,166		2,350
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	380		1,122
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	431		855
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	309		760
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	277		571
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	$ (2)		$ (7)