Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Total revenue	$ 17,181	$ 15,698	$ 13,376
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	8,279	7,666	5,923
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,633	1,335	1,260
U.K.
Disaggregation of Revenue [Line Items]
Total revenue	2,220	1,975	1,819
Ireland
Disaggregation of Revenue [Line Items]
Total revenue	185	146	113
Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	3,153	2,900	2,672
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,711	1,676	1,589
Operating Segments | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	11,290	10,517	9,524
Operating Segments | Risk Capital | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	4,991	4,697	4,067
Operating Segments | Risk Capital | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,131	954	909
Operating Segments | Risk Capital | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	1,409	1,266	1,186
Operating Segments | Risk Capital | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	95	78	61
Operating Segments | Risk Capital | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	2,321	2,176	2,036
Operating Segments | Risk Capital | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,343	1,346	1,265
Operating Segments | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	5,907	5,209	3,864
Operating Segments | Human Capital | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	3,304	2,997	1,868
Operating Segments | Human Capital | Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue	502	381	351
Operating Segments | Human Capital | U.K.
Disaggregation of Revenue [Line Items]
Total revenue	811	709	633
Operating Segments | Human Capital | Ireland
Disaggregation of Revenue [Line Items]
Total revenue	90	68	52
Operating Segments | Human Capital | Europe, Middle East, & Africa other than U.K. and Ireland
Disaggregation of Revenue [Line Items]
Total revenue	832	724	636
Operating Segments | Human Capital | Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	368	330	324
Operating Segments | Commercial Risk Solutions | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	8,497	7,861	7,043
Operating Segments | Reinsurance Solutions | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	2,793	2,656	2,481
Operating Segments | Health Solutions | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	3,839	3,335	2,433
Operating Segments | Wealth Solutions | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	2,068	1,874	1,431
Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(16)	(28)	(12)
Eliminations | Risk Capital
Disaggregation of Revenue [Line Items]
Total revenue	11	16	6
Eliminations | Human Capital
Disaggregation of Revenue [Line Items]
Total revenue	5	12	6
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Total revenue	(16)	(28)	(12)
Corporate/Eliminations | U.S.
Disaggregation of Revenue [Line Items]
Total revenue	(16)	(28)	(12)
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