Loss and Loss Adjustment Expense Reserves (Details) $ in Millions	6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 607	$ 63
Personal Lines \| Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	520	$ 235
Personal Lines \| Personal auto business \| Agency
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of prior year claims and claims adjustment expense		0.60
Personal Lines \| Personal property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	50	$ (30)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	45	(140)
Short-Duration Insurance Contract, Accident Year 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	400
Short-Duration Insurance Contract, Accident Year 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 115	60
Short-Duration Insurance Contract, Accident Year 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ 20