Claim and Claim Adjustment Expense Reserves - Net Prior Year Loss Reserve Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	$ (1)	$ (1)	$ 190	$ 190
Total property & casualty operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(1)	(1)	93	56
Other professional liability and management liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	34	25	104	43
Surety
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(25)	(25)	(51)	(47)
Warranty
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(9)		(9)	10
Commercial auto
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	7		7	50
General liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	115		226	62
Workers’ compensation
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(50)	(1)	(82)	(66)
Other property and casualty operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(73)		(102)	4
Other insurance operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	$ 0	$ 0	$ 97	$ 134