Revenue Recognition and Deferred Revenue (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Contract with Customer, Liability, Revenue Recognized	$ 0	$ 1
Revenue from Contract with Customer, Excluding Assessed Tax	18	14
Contract with Customer, Receivable, after Allowance for Credit Loss	10		$ 9
Contract with Customer, Asset, after Allowance for Credit Loss	100		95
Contract with Customer, Liability	0		1
Variable Interest Entity, Primary Beneficiary [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	5
Contract with Customer, Asset, after Allowance for Credit Loss	88		87
Contract with Customer, Performance Obligation Satisfied in Previous Period	6	5
Managing General Agency Underwriter
Contract with Customer, Asset, after Allowance for Credit Loss	12		$ 7
Employer Stop Loss
Revenue from Contract with Customer, Excluding Assessed Tax	5	5
Affinity Products
Revenue from Contract with Customer, Excluding Assessed Tax	5	5
Commercial Auto
Revenue from Contract with Customer, Excluding Assessed Tax	4	3
Product and Service, Other
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Marine
Revenue from Contract with Customer, Excluding Assessed Tax	1	1
Professional Liability
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2	$ 0
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