Claim and Claim Adjustment Expense Reserves - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophes, net of reinsurance	$ 82	$ 68	$ 170	$ 120
Pretax (favorable) unfavorable development	23	18	16	31
Commercial auto | CNA Financial
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable development	(12)	(17)	(19)	(4)
Other insurance operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable development	35	35	35	35
Other insurance operations | CNA Financial
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable development	$ 35	$ 35	$ 35	$ 35