Condensed Consolidated Statements of Operations (Unaudited) - (Parenthetical) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Income Statement [Abstract]
Insurance claims and policyholders benefits, including policyholders' dividends, remeasurement impact	$ 36	$ 48	$ 59	$ 88

Insurance Claims And Policyholders Benefits, Including Policyholders' Dividends, Remeasurement Impact

No definition available.

Name:	cna_InsuranceClaimsAndPolicyholdersBenefitsIncludingPolicyholdersDividendsRemeasurementImpact
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration