Summary of Significant Accounting Policies - Claim and Claim Adjustment Expense Reserves (Details) - CNA - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Insurance reserves:
Anticipated amounts due from insureds related to losses under deductible policies	$ 1,600	$ 1,300
Interest rate used in determining present value of obligations of structured settlements unfunded by annuities	6.50%	6.60%
Discounted reserves for unfunded structured settlements	$ 432	$ 444
Discounted reserves for unfunded structured settlements, discount amount	505	535
Interest recognized on discounted reserves for unfunded structured settlements	$ 32	$ 33	$ 34
Interest rate used to discount workers' compensation lifetime claim reserves	3.50%	3.50%
Discounted reserves for workers' compensation lifetime claim reserves	$ 167	$ 179
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves	6	6	$ 9
Workers’ compensation
Insurance reserves:
Discounted reserves for workers' compensation lifetime claims reserves, discount amount	$ 78	$ 80