Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Total revenue	$ 15,698	$ 13,376	$ 12,479
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	7,666	5,923	5,666
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,335	1,260	1,137
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,975	1,819	1,660
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	146	113	99
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,900	2,672	2,443
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,676	1,589	1,474
Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	10,517	9,524	8,905
Operating Segments | Risk Capital | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	4,697	4,067	3,910
Operating Segments | Risk Capital | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	954	909	837
Operating Segments | Risk Capital | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,266	1,186	1,068
Operating Segments | Risk Capital | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	78	61	53
Operating Segments | Risk Capital | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,176	2,036	1,877
Operating Segments | Risk Capital | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,346	1,265	1,160
Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	5,209	3,864	3,591
Operating Segments | Human Capital | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	2,997	1,868	1,773
Operating Segments | Human Capital | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	381	351	300
Operating Segments | Human Capital | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	709	633	592
Operating Segments | Human Capital | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	68	52	46
Operating Segments | Human Capital | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	724	636	566
Operating Segments | Human Capital | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	330	324	314
Operating Segments | Commercial Risk Solutions | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	7,861	7,043	6,715
Operating Segments | Reinsurance Solutions | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	2,656	2,481	2,190
Operating Segments | Health Solutions | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	3,335	2,433	2,224
Operating Segments | Wealth Solutions | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	1,874	1,431	1,367
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(28)	(12)	(17)
Eliminations | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	16	6	9
Eliminations | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	12	6	8
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(28)	(12)	(17)
Corporate/Eliminations | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	(28)	(12)	(17)
Corporate/Eliminations | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Corporate/Eliminations | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Corporate/Eliminations | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Corporate/Eliminations | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Corporate/Eliminations | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	$ 0	$ 0	$ 0