Revenue Recognition and Deferred Revenue (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Contract with Customer, Liability, Revenue Recognized		$ 1	$ 0
Revenue from Contract with Customer, Excluding Assessed Tax	$ 13	10	31	$ 25
Contract with Customer, Asset, after Allowance for Credit Loss	101		101		$ 95
Contract with Customer, Liability	0		0		1
Variable Interest Entity, Primary Beneficiary [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3		8
Contract with Customer, Asset, after Allowance for Credit Loss	90		90		87
Contract with Customer, Performance Obligation Satisfied in Previous Period			9	5
Managing General Agency Underwriter
Contract with Customer, Asset, after Allowance for Credit Loss	11		11		$ 7
Employer Stop Loss
Revenue from Contract with Customer, Excluding Assessed Tax	2	2	7	7
Affinity Products
Revenue from Contract with Customer, Excluding Assessed Tax	5	5	10	10
Commercial Auto
Revenue from Contract with Customer, Excluding Assessed Tax	3	2	7	5
Product and Service, Other
Revenue from Contract with Customer, Excluding Assessed Tax	0	0	0	0
Marine
Revenue from Contract with Customer, Excluding Assessed Tax	1	1	2	2
Professional Liability
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2	$ 0	$ 4	$ 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