Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,886	$ 2,497	$ 6,411	$ 5,716
United States
Disaggregation of Revenue [Line Items]
Total revenue	1,263	1,107	2,571	2,334
Americas other than United States
Disaggregation of Revenue [Line Items]
Total revenue	268	219	518	447
United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue	498	444	1,028	944
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	39	21	72	44
Europe, Middle East, & Africa other than United Kingdom and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	499	404	1,590	1,359
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	319	302	632	588
Elimination
Disaggregation of Revenue [Line Items]
Total revenue	(4)	(2)	(11)	(7)
Commercial Risk Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	1,349	1,126	2,638	2,272
Reinsurance Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	500	448	1,422	1,296
Retirement Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	440	393	874	790
Health Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	307	258	843	760
Data & Analytic Services | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue	$ 294	$ 274	$ 645	$ 605