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Commitments, Contingencies, and Guarantees (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Property Lease Guarantee [Member]    
Loss Contingencies [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 41us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_PropertyLeaseGuaranteeMember		  
Guarantee and indemnification agreements [Member]    
Loss Contingencies [Line Items]    
Guarantor Obligations, Current Carrying Value 5us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= cna_GuaranteeandindemnificationagreementsMember		 5us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= cna_GuaranteeandindemnificationagreementsMember
Guarantee Obligations [Member]    
Loss Contingencies [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted 375us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_GuaranteeObligationsMember		  
Indemnification Agreement [Member]    
Loss Contingencies [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted 324us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_IndemnificationGuaranteeMember		  
Structured settlement annuities [Member]    
Loss Contingencies [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 2,000us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= cna_StructuredsettlementannuitiesMember