Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,525	$ 3,219
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,308	1,227
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	250	228
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	530	500
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	33	23
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	1,091	955
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	313	286
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	(7)	(5)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,289	1,146
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	922	848
Retirement Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	434	397
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	536	502
Data & Analytic Services | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 351	$ 331