Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 18	$ 7
Other income, net	3	1
Total revenues	1,258	1,116
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	864	747
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	227	238
Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	90	86
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	46	27
Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	10	10
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1	0
Other income, net	2	1
Total revenues	806	733
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	547	490
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	156	145
Operating Segments | Retail | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	86	82
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	14	15
Operating Segments | Retail | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Operating Segments | Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	5	1
Other income, net	1	0
Total revenues	298	255
Operating Segments | Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	206	158
Operating Segments | Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	49	76
Operating Segments | Programs | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1	2
Operating Segments | Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	26	8
Operating Segments | Programs | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	10	10
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	1	0
Other income, net	0	0
Total revenues	142	123
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	111	99
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	21	18
Operating Segments | Wholesale Brokerage | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3	2
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	6	4
Operating Segments | Wholesale Brokerage | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Other
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	11	6
Other income, net	0	0
Total revenues	12	5
Other | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Other | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1	(1)
Other | Other Supplemental Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Other | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Other | Earned premium
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0	$ 0