Claim and Claim Adjustment Expense Reserves - Net Prior Year Loss Reserve Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Net Prior Year Development [Abstract]
Total pretax unfavorable development	$ 91	$ 108	$ 191	$ 191
Total property & casualty operations
Net Prior Year Development [Abstract]
Total pretax unfavorable development	(6)	(4)	94	57
Other professional liability and management liability
Net Prior Year Development [Abstract]
Total pretax unfavorable development	25	18	70	18
Surety
Net Prior Year Development [Abstract]
Total pretax unfavorable development	(26)	(22)	(26)	(22)
Warranty
Net Prior Year Development [Abstract]
Total pretax unfavorable development				10
Commercial auto
Net Prior Year Development [Abstract]
Total pretax unfavorable development				50
General liability
Net Prior Year Development [Abstract]
Total pretax unfavorable development	56	62	111	62
Workers’ compensation
Net Prior Year Development [Abstract]
Total pretax unfavorable development	(32)	(66)	(32)	(65)
Other property and casualty operations
Net Prior Year Development [Abstract]
Total pretax unfavorable development	(29)	4	(29)	4
Other insurance operations
Net Prior Year Development [Abstract]
Total pretax unfavorable development	$ 97	$ 112	$ 97	$ 134