|**Claim and Claim Adjustment Expense Reserves (Tables)**                                                       | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|-------------------------------------------------------------------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Liability for Claims and Claims Adjustment Expense [Abstract]**                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Net prior year development                                                                                     | The following tables and discussion present the net prior year development.Three months ended March 31, 2016(In millions)SpecialtyCommercialInternationalCorporate& OtherNon-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(34)$(14)$(4)$—$(52)Pretax (favorable) unfavorable premium development(11)(2)(1)—(14)Total pretax (favorable) unfavorable net prior year development$(45)$(16)$(5)$—$(66)Three months ended March 31, 2015(In millions)SpecialtyCommercialInternationalCorporate& OtherNon-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$2$(5)$(4)$—$(7)Pretax (favorable) unfavorable premium development(6)(1)16—9Total pretax (favorable) unfavorable net prior year development$(4)$(6)$12$—$2
|                                                                                                              
|                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Three months ended March 31, 2016                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                                                                  | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Pretax (favorable) unfavorable premium development                                                             | (11                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total pretax (favorable) unfavorable net prior year development                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|                                                                                                              
|                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Three months ended March 31, 2015                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                                                                  | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Pretax (favorable) unfavorable premium development                                                             | (6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total pretax (favorable) unfavorable net prior year development                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment          | The following table presents further detail of the net prior year claim and allocated claim adjustment expense reserve development (development) recorded for the Specialty segment.Three months ended March 31(In millions)20162015Pretax (favorable) unfavorable development:Medical Professional Liability$(7)$14Other Professional Liability and Management Liability(9)(3)Surety—1Warranty2—Other(20)(10)Total pretax (favorable) unfavorable development$(34)$2                                                                                                                                                                                                                                                                                                                                                                                            
|                                                                                                              
|                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Three months ended March 31                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                                                                  | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Pretax (favorable) unfavorable development:                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Medical Professional Liability                                                                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Other Professional Liability and Management Liability                                                          | (9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Surety                                                                                                         | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Warranty                                                                                                       | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Other                                                                                                          | (20                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total pretax (favorable) unfavorable development                                                               | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment         | The following table presents further detail of the development recorded for the Commercial segment.Three months ended March 31(In millions)20162015Pretax (favorable) unfavorable development:Commercial Auto$(15)$—General Liability(15)4Workers' Compensation4(1)Property and Other12(8)Total pretax (favorable) unfavorable development$(14)$(5)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|                                                                                                              
|                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Three months ended March 31                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                                                                  | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Pretax (favorable) unfavorable development:                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Commercial Auto                                                                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|General Liability                                                                                              | (15                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Workers' Compensation                                                                                          | 4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Property and Other                                                                                             | 12                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total pretax (favorable) unfavorable development                                                               | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Net prior year claim and allocated claim adjustment expense reserve development for International segment      | The following table presents further detail of the development recorded for the International segment.Three months ended March 31(In millions)20162015Pretax (favorable) unfavorable development:Medical Professional Liability$—$—Other Professional Liability(1)—Liability—(5)Property & Marine(4)(6)Other17Total pretax (favorable) unfavorable development$(4)$(4)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                                                                              
|                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Three months ended March 31                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                                                                  | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Pretax (favorable) unfavorable development:                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Medical Professional Liability                                                                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Other Professional Liability                                                                                   | (1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Liability                                                                                                      | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Property & Marine                                                                                              | (4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other                                                                                                          | 1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Total pretax (favorable) unfavorable development                                                               | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Impact Of Loss Portfolio Transfer On Consolidated Statements Of Operations [Table Text Block]                  | The following table presents the impact of the Loss Portfolio Transfer on the Condensed Consolidated Statements of Operations.Three months ended March 31(In millions)20162015Net A&EP adverse development before consideration of LPT$200$—Provision for uncollectible third-party reinsurance on A&EP——Additional amounts ceded under LPT200—Retroactive reinsurance benefit recognized(73)(5)Pretax impact of unrecognized deferred retroactive reinsurance benefit$127$(5)                                                                                                                                                                                                                                                                                                                                                                                   
|                                                                                                              
|                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Three months ended March 31                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                                                                  | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net A&EP adverse development before consideration of LPT                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Provision for uncollectible third-party reinsurance on A&EP                                                    | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Additional amounts ceded under LPT                                                                             | 200                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Retroactive reinsurance benefit recognized                                                                     | (73                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Pretax impact of unrecognized deferred retroactive reinsurance benefit                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

X- DefinitionImpact of the loss portfolio transfer on the consolidated statements of operations
+ ReferencesNo definition available.
+ Details**Name:**
cna_ImpactOfLossPortfolioTransferOnConsolidatedStatementsOfOperationsTableTextBlock**Namespace Prefix:**
cna_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionPretax Favorable Unfavorable Net Prior Year Claim And Allocated Claim Adjustment Expense Reserve Development [Table Text Block]
+ ReferencesNo definition available.
+ Details**Name:**
cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveAndPremiumDevelopmentTableTextBlock**Namespace Prefix:**
cna_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionNet prior year claim and allocated claim adjustment expense reserve development for International segment
+ ReferencesNo definition available.
+ Details**Name:**
cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentForInternationalSegmentTableTextBlock**Namespace Prefix:**
cna_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionNet prior year claim and allocated claim adjustment expense reserve development for Commercial segment
+ ReferencesNo definition available.
+ Details**Name:**
cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentforCommercialsegmentTableTextBlock**Namespace Prefix:**
cna_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionNet prior year claim and allocated claim adjustment expense reserve development for Specialty segment
+ ReferencesNo definition available.
+ Details**Name:**
cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentforSpecialtysegmentTableTextBlock**Namespace Prefix:**
cna_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- ReferencesNo definition available.
+ Details**Name:**
us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseAbstract**Namespace Prefix:**
us-gaap_**Data Type:**
xbrli:stringItemType**Balance Type:**
na**Period Type:**
duration