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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Increase (decrease) in claims and claims expense
Prior years	$ 32	$ 183	$ (322)	$ 380
Auto
Increase (decrease) in claims and claims expense
Prior years	(65)	33	(319)	105
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(12)	62	(392)	136
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	51	(14)	164	(8)
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	1	19	159	51
Other business lines
Increase (decrease) in claims and claims expense
Prior years	(3)	1	1	12
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	59	82	65	85
Protection Services
Increase (decrease) in claims and claims expense
Prior years	1	0	0	(1)
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(14)	17	(314)	6
Catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(10)	6	(26)	(41)
Catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(1)	16	(278)	61
Catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(3)	(11)	(5)	(23)
Catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	0	6	(5)	9
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
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	46	166	(8)	374
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(55)	27	(293)	146
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(11)	46	(114)	75
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	54	(3)	169	15
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	1	13	164	42
Non-catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	(3)	1	1	12
Non-catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	59	82	65	85
Non-catastrophe losses | Protection Services
Increase (decrease) in claims and claims expense
Prior years	$ 1	$ 0	$ 0	$ (1)