Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 93	$ 52	$ 7
Other income, net	7	6	3
Total revenues	4,805	4,257	3,573
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3,282	2,868	2,377
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	992	995	912
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	188	178	159
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	166	130	89
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	77	28	26
Operating Segments
Disaggregation Of Revenue [Line Items]
Total revenues	4,745	4,222	3,568
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	6	1	0
Other income, net	3	4	3
Total revenues	2,729	2,508	2,157
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,874	1,692	1,421
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	631	597	546
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	171	164	138
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	44	50	49
Operating Segments | Retail | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0
Operating Segments | Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	23	12	1
Other income, net	2	1	0
Total revenues	1,400	1,173	958
Operating Segments | Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	924	736	589
Operating Segments | Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	270	322	298
Operating Segments | Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	9	9	16
Operating Segments | Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	95	65	28
Operating Segments | Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	77	28	26
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	6	2	0
Other income, net	0	0	0
Total revenues	616	541	453
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	483	440	367
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	92	78	69
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8	6	5
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	27	15	12
Operating Segments | Wholesale Brokerage | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0
Other
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	58	37	6
Other income, net	2	1	0
Total revenues	60	35	5
Other | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1	0	0
Other | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(1)	(2)	(1)
Other | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	1	0
Other | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0
Other | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0	$ 0	$ 0