Summary of Significant Accounting Policies - Claim and Claim Adjustment Expense Reserves (Details) - CNA - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Insurance reserves:
Anticipated amounts due from insureds related to losses under deductible policies	$ 1,200	$ 1,100
Interest rate used in determining present value of obligations of structured settlements unfunded by annuities	6.40%	6.40%
Discounted reserves for unfunded structured settlements	$ 465	$ 485
Discounted reserves for unfunded structured settlements, discount amount	559	590
Interest recognized on discounted reserves for unfunded structured settlements	$ 34	$ 36	$ 36
Interest rate used to discount workers' compensation lifetime claim reserves	3.50%	3.50%
Discounted reserves for workers' compensation lifetime claim reserves	$ 196	$ 211
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves	9	9	$ 12
Workers’ compensation
Insurance reserves:
Discounted reserves for workers' compensation lifetime claims reserves, discount amount	$ 88	$ 93