Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,606	$ 2,561	$ 5,749	$ 5,651
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,146	1,125	2,307	2,241
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	241	243	467	480
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	400	413	852	897
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	501	493	1,510	1,472
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	318	287	613	561
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,167	1,166	2,285	2,350
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	420	380	1,208	1,122
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	419	431	839	855
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	317	309	803	760
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	286	277	622	571
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	$ (3)	$ (2)	$ (8)	$ (7)