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
	Mar. 31, 2024	Mar. 31, 2023
Increase (decrease) in claims and claims expense
Prior years	$ (151)	$ (15)
Auto
Increase (decrease) in claims and claims expense
Prior years	(74)	(25)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(189)	(20)
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	52	3
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	51	24
Other business lines
Increase (decrease) in claims and claims expense
Prior years	5	1
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	4	2
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(162)	(42)
Catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(7)	(28)
Catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(149)	(8)
Catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(3)	(7)
Catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(3)	1
Catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	0	0
Catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	0	0
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	11	27
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(67)	3
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(40)	(12)
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	55	10
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	54	23
Non-catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	5	1
Non-catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	$ 4	$ 2