Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 22	$ 10	$ 40	$ 17
Other income, net	2	1	5	2
Total revenues	1,178	1,047	2,435	2,163
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	821	709	1,687	1,456
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	240	252	463	489
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	39	30	130	117
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	36	33	82	60
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	18	12	28	22
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1	0	2	0
Other income, net	1	1	2	2
Total revenues	646	591	1,452	1,324
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	449	397	996	885
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	156	151	312	296
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	32	27	119	111
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7	15	21	30
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	5	2	10	3
Other income, net	1	0	2	0
Total revenues	359	310	657	565
Operating Segments | Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	243	197	449	356
Operating Segments | Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	61	82	110	157
Operating Segments | Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	6	2	7	4
Operating Segments | Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	25	15	51	23
Operating Segments | Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	18	12	28	22
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1	0	2	0
Other income, net	0	0	0	0
Total revenues	159	139	301	262
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	129	115	242	215
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	24	20	43	38
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1	1	4	2
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	4	3	10	7
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	15	8	26	14
Other income, net	0	0	1	0
Total revenues	14	7	25	12
Other | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0	0	0
Other | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(1)	(1)	(2)	(2)
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