Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Total revenue	$ 5,034	$ 4,729
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	2,010	2,002
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	384	374
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	689	621
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	53	45
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	1,489	1,262
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	409	425
Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	3,502	3,191
Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,539	1,545
Operating Segments | U.S. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,326	1,221
Operating Segments | U.S. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	691	788
Operating Segments | Americas other than U.S. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	261	254
Operating Segments | Americas other than U.S. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	123	120
Operating Segments | U.K. | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	482	430
Operating Segments | U.K. | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	207	191
Operating Segments | Ireland | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	25	22
Operating Segments | Ireland | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	28	23
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,119	946
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	370	316
Operating Segments | Asia Pacific | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	289	318
Operating Segments | Asia Pacific | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	120	107
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(7)	(7)
Eliminations | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	4	5
Eliminations | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	3	2
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(7)	(7)
Corporate/Eliminations | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	(7)	(7)
Corporate/Eliminations | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate/Eliminations | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate/Eliminations | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate/Eliminations | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate/Eliminations | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Commercial Risk Solutions | Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	2,223	2,002
Reinsurance Solutions | Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,279	1,189
Health Solutions | Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,119	1,026
Wealth Solutions | Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	$ 420	$ 519