Condensed Consolidated Statements of Operations (Unaudited) - (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Income Statement [Abstract]
Insurance claims and policyholders benefits, including policyholders' dividends, remeasurement impact	$ (15)	$ 1

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:	debit
Period Type:	duration