Claim and Claim Adjustment Expense Reserves - Excess Workers' Compensation LPT (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Feb. 05, 2021	Mar. 31, 2021	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jan. 01, 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reinsurance recoverable			$ 6,051	$ 5,412
Claims paid			5,189	$ 4,601	$ 4,302
Outstanding reserves maintained as security for obligations			298
CNA | Loss Portfolio Transfer for Workers' Compensation Liabilities
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reinsurance recoverable			$ 690			$ 690
Payment for reinsurance premium	$ 697
Claims paid	$ 64
Loss contingency, after tax and noncontrolling interests		$ 11
CNA | Loss Portfolio Transfer for Workers' Compensation Liabilities | Maximum
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reinsurance recoverable						$ 1,000