Revenue Recognition and Deferred Revenue (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Contract with Customer, Liability, Revenue Recognized		$ 1	$ 1
Revenue from Contract with Customer, Excluding Assessed Tax	$ 23	15	54	$ 39
Contract with Customer, Asset, after Allowance for Credit Loss	140		140		$ 95
Contract with Customer, Liability	0		0		1
Variable Interest Entity, Primary Beneficiary [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2		10
Contract with Customer, Asset, after Allowance for Credit Loss	95		95		87
Contract with Customer, Performance Obligation Satisfied in Previous Period			10	5
Managing General Agency Underwriter
Contract with Customer, Asset, after Allowance for Credit Loss	45		45		$ 7
Marine and Energy
Revenue from Contract with Customer, Excluding Assessed Tax	1	1	2	2
Accident & Health
Revenue from Contract with Customer, Excluding Assessed Tax	7	8	24	25
Other Property
Revenue from Contract with Customer, Excluding Assessed Tax	2	0	2	0
Reinsurer, Other
Revenue from Contract with Customer, Excluding Assessed Tax	1	0	1	0
Directors and Officers Liability Insurance
Revenue from Contract with Customer, Excluding Assessed Tax	0	0	0	0
Miscellaneous Specialty
Revenue from Contract with Customer, Excluding Assessed Tax	1	0	1	0
Specialty Automobile
Revenue from Contract with Customer, Excluding Assessed Tax	7	4	14	10
Niche Specialty Risks
Revenue from Contract with Customer, Excluding Assessed Tax	2	0	2	0
Other Professional
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2	$ 1	$ 7	$ 1
Minimum [Member]
Employee Stop Loss Time When Base Commissions Recognized Begin and End In Months	17
Maximum [Member]
Employee Stop Loss Time When Base Commissions Recognized Begin and End In Months	20