Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,090	$ 2,381
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,116
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	237
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	484
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	979
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	274
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,184
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	742
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	424
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	451
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	294
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	$ (5)