Summary of Significant Accounting Policies - Claim and Claim Adjustment Expense Reserves (Details) - CNA - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Insurance reserves:
Anticipated amounts due from insureds related to losses under deductible policies	$ 1,100	$ 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	$ 485	$ 503
Discounted reserves for unfunded structured settlements, discount amount	590	621
Interest recognized on discounted reserves for unfunded structured settlements	$ 36	$ 36	$ 35
Interest rate used to discount workers' compensation lifetime claim reserves	3.50%	3.50%
Discounted reserves for workers' compensation lifetime claim reserves	$ 211	$ 228
Discounted reserves for workers' compensation lifetime claims reserves, discount amount	93	97
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves	$ 9	$ 12	$ 15
Interest rate used to discount long term care claim reserves	5.90%	5.80%
Discounted reserves for long term care claim reserves	$ 2,800	$ 2,700
Discounted reserves for long term care claim reserves, discount amount	$ 450	$ 428