Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 2,497	$ 2,606	$ 5,716	$ 5,749
United States
Disaggregation of Revenue [Line Items]
Revenue	1,107	1,146	2,334	2,307
Americas other than United States
Disaggregation of Revenue [Line Items]
Revenue	219	241	447	467
United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	444	400	944	852
Ireland
Disaggregation of Revenue [Line Items]
Revenue	21	20	44	42
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Revenue	404	481	1,359	1,468
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	302	318	588	613
Elimination
Disaggregation of Revenue [Line Items]
Revenue	(2)	(3)	(7)	(8)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,126	1,167	2,272	2,285
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	448	420	1,296	1,208
Retirement Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	393	419	790	839
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	258	317	760	803
Data & Analytic Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 274	$ 286	$ 605	$ 622