Insurance Contracts (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Insurance [Line Items]
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 41
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	186	$ 115	$ 56	$ 33
Liability for Claims and Claims Adjustment Expense	893	805
Short Duration Insurance Contract Liability For Unpaid Clams and Allocated Claim Adjustment Expense Subtotal For Separately Presented	22	0
Other adjustments (including foreign exchange)	4	(12)	(4)
Commercial Auto | Operating Segments
Insurance [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	85
Liability for Claims and Claims Adjustment Expense	107
Unallocated Loss Adjustment Expense Reserve | Segment Reconciling Items
Insurance [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	2
Liability for Claims and Claims Adjustment Expense	6
Other Insurance Contracts | Segment Reconciling Items
Insurance [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	69
Liability for Claims and Claims Adjustment Expense	84	44
Specialty Property & Casualty Program
Insurance [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	156
Specialty Property & Casualty Program | Operating Segments
Insurance [Line Items]
Liability for Claims and Claims Adjustment Expense	197
Specialty Property & Casualty Program | Commercial Auto
Insurance [Line Items]
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	22
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	28
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 6
Short-Duration Insurance Contracts, Historical Claims Duration, Year One	9.30%
Short-Duration Insurance Contracts, Historical Claims Duration, Year Two	2.90%
Short-Duration Insurance Contracts, Historical Claims Duration, Year Three	10.70%
Specialty Property & Casualty Program | Commercial Auto | Short-Duration Insurance Contract, Accident Year 2021
Insurance [Line Items]
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 1	0	0
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	0	0	0
Short-Duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-Duration Insurance Contract, Cumulative Number of Reported Claims	75
Specialty Property & Casualty Program | Commercial Auto | Short-Duration Insurance Contract, Accident Year 2022
Insurance [Line Items]
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 8	8
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2	1
Short-Duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3
Short-Duration Insurance Contract, Cumulative Number of Reported Claims	1,112
Specialty Property & Casualty Program | Commercial Auto | Short-Duration Insurance Contract, Accident Year 2023
Insurance [Line Items]
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 19
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	4
Short-Duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10
Short-Duration Insurance Contract, Cumulative Number of Reported Claims	2,531
Specialty Property & Casualty Program | Commercial Auto | Operating Segments
Insurance [Line Items]
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 22
Specialty Property & Casualty Program | Unallocated Loss Adjustment Expense Reserve | Segment Reconciling Items
Insurance [Line Items]
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	5
Specialty Property & Casualty Program | Other Insurance Contracts
Insurance [Line Items]
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	14
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	16
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2
Short Duration Insurance Contract Liability For Unpaid Clams and Allocated Claim Adjustment Expense Subtotal For Separately Presented	$ 14
Short-Duration Insurance Contracts, Historical Claims Duration, Year One	3.10%
Short-Duration Insurance Contracts, Historical Claims Duration, Year Two	0.90%
Short-Duration Insurance Contracts, Historical Claims Duration, Year Three	0.00%
Specialty Property & Casualty Program | Other Insurance Contracts | Short-Duration Insurance Contract, Accident Year 2021
Insurance [Line Items]
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 0	0	0
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	0	0	$ 0
Short-Duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-Duration Insurance Contract, Cumulative Number of Reported Claims	0
Specialty Property & Casualty Program | Other Insurance Contracts | Short-Duration Insurance Contract, Accident Year 2022
Insurance [Line Items]
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 0	0
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	0	$ 0
Short-Duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-Duration Insurance Contract, Cumulative Number of Reported Claims	646
Specialty Property & Casualty Program | Other Insurance Contracts | Short-Duration Insurance Contract, Accident Year 2023
Insurance [Line Items]
Short-Duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 16
Short-Duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2
Short-Duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 8
Short-Duration Insurance Contract, Cumulative Number of Reported Claims	11,595
Specialty Property & Casualty Program | Other Insurance Contracts | Segment Reconciling Items
Insurance [Line Items]
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 14