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, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Increase (decrease) in claims and claims expense
Prior years	$ (753)	$ (424)	$ (2,398)	$ (1,045)
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	10	(28)	73	(38)
Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(763)	(396)	(2,471)	(1,007)
Auto
Increase (decrease) in claims and claims expense
Prior years	(884)	(483)	(2,372)	(1,163)
Auto \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(8)	(3)	(19)	(30)
Auto \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(876)	(480)	(2,353)	(1,133)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(49)	(54)	(123)	(32)
Homeowners \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	19	(14)	71	3
Homeowners \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(68)	(40)	(194)	(35)
Specialty lines
Increase (decrease) in claims and claims expense
Prior years	12	66	(36)	142
Specialty lines \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	0	(6)	22	(11)
Specialty lines \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	12	72	(58)	153
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(38)	(85)	(66)	(104)
Commercial lines \| Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(1)	(5)	(1)	0
Commercial lines \| Excluding catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(37)	(80)	(65)	(104)
Brokered solutions and collateral protection
Increase (decrease) in claims and claims expense
Prior years	(10)	(14)	(17)	(39)
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	(10)	(14)	(17)	(39)
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	216	146	216	151
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	216	146	$ 216	$ 151
Run-off Property-Liability \| Annual Reserve Review
Increase (decrease) in claims and claims expense
Prior years	$ 216	$ 146