Summary of Significant Accounting Policies, Claim and Claim Adjustment Expense Reserves (Details) - CNA [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Claim, Claim Adjustment Expense and Future Policy Benefit Reserves [Abstract]
Anticipated amounts due from insureds related to losses under deductible policies	$ 1,200	$ 1,200
Interest rate used in determining present value of obligations of structured settlements unfunded by annuities	6.50%	7.10%
Discounted reserves for unfunded structured settlements	$ 520	$ 497
Discounted reserves for unfunded structured settlements, discount amount	657	724
Interest recognized on discounted reserves for unfunded structured settlements	$ 35	$ 36	$ 40
Interest rate used to discount workers' compensation lifetime claim reserves	3.50%	3.50%
Discounted reserves for workers' compensation lifetime claim reserves	$ 258	$ 293
Discounted reserves for workers' compensation lifetime claims reserves, discount amount	113	135
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves	$ 15	$ 21	$ 16
Interest rate used to discount long term care claim reserves	5.80%	5.90%
Discounted reserves for long term care claim reserves	$ 2,700	$ 2,700
Discounted reserves for long term care claim reserves, discount amount	$ 439	$ 462