Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 35	$ 22	$ 53	$ 40
Other income, net	1	2	2	5
Total revenues	1,285	1,178	2,689	2,435
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	878	821	1,836	1,687
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	268	240	533	463
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	41	39	141	130
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	45	36	88	82
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17	18	36	28
Operating Segments
Disaggregation Of Revenue [Line Items]
Total revenues	1,260	1,164	2,654	2,410
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	3	1	4	2
Other income, net	0	1	1	2
Total revenues	697	646	1,604	1,452
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	483	449	1,099	996
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	173	156	350	312
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	31	32	128	119
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7	7	22	21
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	5	11	10
Other income, net	1	1	1	2
Total revenues	381	359	709	657
Operating Segments | Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	254	243	473	449
Operating Segments | Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	66	61	131	110
Operating Segments | Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7	6	8	7
Operating Segments | Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	30	25	49	51
Operating Segments | Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17	18	36	28
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1	1	2	2
Other income, net	0	0	0	0
Total revenues	182	159	341	301
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	141	129	264	242
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	29	24	53	43
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3	1	5	4
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8	4	17	10
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	25	15	36	26
Other income, net	0	0	0	1
Total revenues	25	14	35	25
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	0	(1)	(1)	(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