|**Claim and Claim Adjustment Expense Reserves (Tables)**                                                             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
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|**Claim and Claim Adjustment Expense Reserves [Abstract]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Net prior year development                                                                                           | Net Prior Year DevelopmentThree months ended March 31, 2014(In millions)CNASpecialtyCNA CommercialHardyCorporate& OtherNon-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(2)$17$10$—$25Pretax (favorable) unfavorable premium development(8)(19)(4)—(31)Total pretax (favorable) unfavorable net prior year development$(10)$(2)$6$—$(6)Three months ended March 31, 2013(In millions)CNASpecialtyCNA CommercialHardyCorporate& OtherNon-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(15)$(11)$(1)$1$(26)Pretax (favorable) unfavorable premium development(8)(10)41(13)Total pretax (favorable) unfavorable net prior year development$(23)$(21)$3$2$(39)
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|Three months ended March 31, 2014                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|(In millions)                                                                                                        | CNASpecialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Pretax (favorable) unfavorable premium development                                                                   | (8                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total pretax (favorable) unfavorable net prior year development                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|Three months ended March 31, 2013                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|(In millions)                                                                                                        | CNASpecialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Pretax (favorable) unfavorable premium development                                                                   | (8                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total pretax (favorable) unfavorable net prior year development                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Net prior year claim and allocated claim adjustment expense reserve development for CNA Specialty segment            | Three months ended March 31(In millions)20142013Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:Medical Professional Liability$1$(3)Other Professional Liability and Management Liability(6)(1)Surety11Warranty——Other2(12)Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(2)$(15)                                                                                                                                                                                                                                                                                                                                                                                         
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|Three months ended March 31                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|(In millions)                                                                                                        | 2014                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Medical Professional Liability                                                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other Professional Liability and Management Liability                                                                | (6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Surety                                                                                                               | 1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Warranty                                                                                                             | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other                                                                                                                | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Net prior year claim and allocated claim adjustment expense reserve development for CNA Commercial segment           | Three months ended March 31(In millions)20142013Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:Commercial Auto$20$(5)General Liability(5)(21)Workers' Compensation1025Property and Other(8)(10)Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$17$(11)                                                                                                                                                                                                                                                                                                                                                                                                                     
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|Three months ended March 31                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|(In millions)                                                                                                        | 2014                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Commercial Auto                                                                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|General Liability                                                                                                    | (5                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Workers' Compensation                                                                                                | 10                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Property and Other                                                                                                   | (8                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Net prior year claim and allocated claim adjustment expense reserve development for Hardy segment                    | Three months ended March 31(In millions)20142013Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:Marine and Aviation$5$(1)Non-Marine Property(2)—Property Treaty(2)—Specialty(1)—Commutations10—Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$10$(1)                                                                                                                                                                                                                                                                                                                                                                                                                       
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|Three months ended March 31                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|(In millions)                                                                                                        | 2014                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Marine and Aviation                                                                                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Non-Marine Property                                                                                                  | (2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Property Treaty                                                                                                      | (2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Specialty                                                                                                            | (1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Commutations                                                                                                         | 10                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

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