Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,379	$ 2,349	$ 8,128	$ 8,000
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,209	1,215	3,516	3,456
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	256	203	723	683
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	358	264	1,210	1,161
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	283	399	1,793	1,871
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	273	268	886	829
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,057	1,029	3,342	3,379
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	291	279	1,499	1,401
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	484	501	1,323	1,356
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	279	278	1,082	1,038
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	271	263	893	834
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	$ (3)	$ (1)	$ (11)	$ (8)