Reserve for Property and Casualty Insurance Claims and Claims Expense - Schedule of Prior Year Reserve Re-Estimates Included in Claims and Claims Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Increase (decrease) in claims and claims expense
Prior years	$ (641)	$ (370)	$ (1,645)	$ (621)
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	51	6	63	(10)
Nationwide Reinsurance Program
Increase (decrease) in claims and claims expense
Prior years			60
Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(692)	(376)	(1,708)	(611)
Auto
Increase (decrease) in claims and claims expense
Prior years	(648)	(431)	(1,488)	(680)
Auto \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(9)	(16)	(11)	(27)
Auto \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(639)	(415)	(1,477)	(653)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	38	30	(74)	22
Homeowners \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	66	18	52	17
Homeowners \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(28)	12	(126)	5
Specialty lines
Increase (decrease) in claims and claims expense
Prior years	(19)	30	(48)	76
Specialty lines \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(3)	2	22	(5)
Specialty lines \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(16)	28	(70)	81
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(11)	9	(28)	(19)
Commercial lines \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(3)	2	0	5
Commercial lines \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(8)	7	(28)	(24)
Brokered solutions and collateral protection
Increase (decrease) in claims and claims expense
Prior years	(1)	(10)	(7)	(25)
Brokered solutions and collateral protection \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	0	0	0	0
Brokered solutions and collateral protection \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(1)	(10)	(7)	(25)
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	0	2	0	5
Run-off Property-Liability \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	0	0	0	0
Run-off Property-Liability \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	$ 0	$ 2	$ 0	$ 5