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		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Increase (decrease) in claims and claims expense
Prior years	$ (424)	$ 32	$ (1,045)	$ (322)
Auto
Increase (decrease) in claims and claims expense
Prior years	(483)	(65)	(1,163)	(319)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(54)	(12)	(32)	(392)
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	66	51	142	164
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(85)	1	(104)	159
Other business lines
Increase (decrease) in claims and claims expense
Prior years	(14)	(3)	(39)	1
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	146	59	151	65
Protection Services
Increase (decrease) in claims and claims expense
Prior years	0	1	0	0
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(28)	(14)	(38)	(314)
Catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(3)	(10)	(30)	(26)
Catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(14)	(1)	3	(278)
Catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(6)	(3)	(11)	(5)
Catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(5)	0	0	(5)
Catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	0	0	0	0
Catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	0	0	0	0
Catastrophe losses | Protection Services
Increase (decrease) in claims and claims expense
Prior years	0	0	0	0
Nationwide Reinsurance Program
Increase (decrease) in claims and claims expense
Prior years			69
2021 Colorado Marshall Fire
Increase (decrease) in claims and claims expense
Prior years			54
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(396)	46	(1,007)	(8)
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(480)	(55)	(1,133)	(293)
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(40)	(11)	(35)	(114)
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	72	54	153	169
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(80)	1	(104)	164
Non-catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	(14)	(3)	(39)	1
Non-catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	146	59	151	65
Non-catastrophe losses | Protection Services
Increase (decrease) in claims and claims expense
Prior years	0	$ 1	0	0
Favorable Severity Development | Personal auto injury coverage
Increase (decrease) in claims and claims expense
Prior years	284		565
Favorable Severity Development | Personal auto physical damage coverage
Increase (decrease) in claims and claims expense
Prior years	$ 196		568
Annual Reserve Review | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years			$ 146	$ 58