Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 31	$ 17	$ 71	$ 34
Other income, net	0	2	6	4
Total revenues	1,186	1,068	3,622	3,231
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	822	721	2,505	2,176
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	251	258	716	748
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	32	31	162	147
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	27	27	110	88
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	23	12	52	34
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	2	0	3	0
Other income, net	0	2	3	4
Total revenues	641	602	2,093	1,926
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	444	412	1,440	1,297
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	158	150	469	446
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	29	29	148	139
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8	9	30	40
Operating Segments | Retail | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0	0
Operating Segments | Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	6	4	17	7
Other income, net	0	0	2	0
Total revenues	353	305	1,010	870
Operating Segments | Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	249	189	696	543
Operating Segments | Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	67	87	177	246
Operating Segments | Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1	1	8	5
Operating Segments | Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7	12	58	35
Operating Segments | Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	23	12	52	34
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	2	1	4	1
Other income, net	0	0	0	0
Total revenues	171	150	472	412
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	129	121	369	336
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	26	21	71	59
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2	1	6	3
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	12	6	22	13
Operating Segments | Wholesale Brokerage | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0	0
Other
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	21	12	47	26
Other income, net	0	0	1	0
Total revenues	21	11	47	23
Other | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	(1)	0	0
Other | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	(1)	(3)
Other | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0	0
Other | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0	0
Other | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0	$ 0	$ 0	$ 0