Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,385	$ 2,379	$ 8,101	$ 8,128
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,169	1,209	3,503	3,516
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	200	256	647	723
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	332	358	1,276	1,210
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	19	17	63	59
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	372	266	1,731	1,734
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	293	273	881	886
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	(6)	(3)	(13)	(11)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,042	1,057	3,314	3,342
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	321	291	1,617	1,499
Retirement Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	468	484	1,258	1,323
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	282	279	1,042	1,082
Data & Analytic Services | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 278	$ 271	$ 883	$ 893