Claim and Claim Adjustment Expense Reserves - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophes, net of reinsurance	$ 88	$ 52
Pretax (favorable) unfavorable development	(7)	13
Commercial auto | CNA Financial
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable development	$ (7)	$ 13