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, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Property and casualty operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	$ (4)	$ (12)	$ 57	$ (19)
Property and casualty operations \| CNA Financial
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(4)	(12)	57	(19)
Medical professional liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		(2)		(2)
Other professional liability and management liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	18	17	18	17
Surety
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(22)	(8)	(22)	(26)
Warranty
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development			10	13
Commercial auto
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development		21	50	21
General liability
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	62	19	62	19
Workers’ compensation
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	(66)	(47)	(65)	(49)
Other property and casualty operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	4	(12)	4	(12)
Other insurance operations
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	112	35	134	35
Total pretax (favorable) unfavorable development \| CNA Financial \| Operating Segments
Net Prior Year Development [Abstract]
Total pretax (favorable) unfavorable development	$ 108	$ 23	$ 191	$ 16