Benefit Plans (Components of Net Periodic Cost (Benefit)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 9	$ 9	$ 17	$ 18
Expected return on plan assets	(14)	(13)	(28)	(27)
Amortization of net actuarial loss (gain)	1	2	3	4
Total net periodic pension cost (benefit)	(4)	(2)	(8)	(5)
Net periodic defined benefits expense (reversal of expense), excluding service cost component	(4)	(2)	(8)	(5)
Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating
Defined Benefit Plan Disclosure [Line Items]
Net periodic defined benefits expense (reversal of expense), excluding service cost component	(3)	(2)	(6)	(4)
Location, Statement of Income, Balance [Axis]: cna:Insuranceclaimsandpolicyholdersbenefitsincludingpolicyholdersdividends
Defined Benefit Plan Disclosure [Line Items]
Net periodic defined benefits expense (reversal of expense), excluding service cost component	$ (1)	$ 0	$ (2)	$ (1)