Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 1,002	$ 607
Personal Lines \| Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	845	520
Personal Lines \| Personal property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		50
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	140	45
Short-Duration Insurance Contract, Accident Year 2025
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	670
Short-Duration Insurance Contract, Accident Year 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 220	400
Short-Duration Insurance Contract, Accident Year 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ 115