Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenue	$ 10,770	$ 10,770	$ 9,998	$ 9,409
Operating Segments | United States
Disaggregation of Revenue [Line Items]
Total revenue		4,677	4,425	3,981
Operating Segments | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue		940	976	899
Operating Segments | United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue		1,555	1,436	1,354
Operating Segments | Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue		2,413	2,025	1,760
Operating Segments | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue		$ 1,185	1,136	1,415
Operating Segments | Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue	4,652		4,169	3,929
Operating Segments | Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,563		1,429	1,361
Operating Segments | Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,865		1,755	1,707
Operating Segments | Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue	1,596		1,515	1,370
Operating Segments | Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue	1,105		1,140	1,050
Intersegment elimination
Disaggregation of Revenue [Line Items]
Total revenue	$ (11)		$ (10)	$ (8)