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, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Total pretax (favorable) unfavorable development
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	$ (1)	$ 17	$ 190	$ 33
Property and casualty operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(1)	(5)	56	(24)
Medical professional liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development				(2)
Other professional liability and management liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	25	11	43	28
Surety
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(25)	(20)	(47)	(46)
Warranty
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		7	10	20
Commercial auto
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		25	50	46
General liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		28	62	47
Workers’ compensation
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(1)	(57)	(66)	(106)
Other property and casualty operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		1	4	(11)
Other insurance operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		$ 22	$ 134	$ 57