Claim and Claim Adjustment Expense Reserves - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophes, net of reinsurance	$ 94	$ 114	$ 214	$ 171
Pretax (favorable) unfavorable development	13	(17)	44	(2)
Commercial Property and Casualty Operations | CNA Financial
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable development	(7)	(17)	(11)	(66)
Other Insurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable development	$ 20		$ 55	64
Hurricane
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophes, net of reinsurance		$ 87		$ 87