Revenue from Contract with Customer (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 95	$ 5
Contract with Customer, Liability	1	1
Contract with Customer, Liability, Revenue Recognized	1	1	$ 1
Contract with Customer, Receivable, after Allowance for Credit Loss	10	7
Revenue from Contract with Customer, Excluding Assessed Tax	51	31	26
Contract with Customer, Performance Obligation Satisfied in Previous Period	5	6	8
Variable Interest Entity [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract with Customer, Asset, after Allowance for Credit Loss	87
Contract with Customer, Liability	0
Employer Stop Loss
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	11	9	8
Affinity Products
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	22	19	18
Commercial Auto
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	12	2
Marine
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3	0
Professional Liability
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3
Product and Service, Other
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1	$ 0	$ 0
Minimum [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Employee Stop Loss Time When Base Commissions Recognized Begin and End In Months	17
Maximum [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Employee Stop Loss Time When Base Commissions Recognized Begin and End In Months	20