|**Debt (Tables)**                                                                    | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|**Debt [Abstract]**                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Schedule of debt instruments                                                         | DebtDecember 31(In millions)20122011Short term debt:Senior notes:8.375%, face amount of $70, due August 15, 2012$—$70Other debt1313Total short term debt1383Long term debt:Senior notes of CNAF:5.850%, face amount of $549, due December 15, 20145485486.500%, face amount of $350, due August 15, 20163483486.950%, face amount of $150, due January 15, 20181491497.350%, face amount of $350, due November 15, 20193483485.875%, face amount of $500, due August 15, 20204964955.750%, face amount of $400, due August 15, 2021397396Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023241241Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 203630Total long term debt2,5572,525Total debt$2,570$2,608
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|December 31                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|(In millions)                                                                        | 2012                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Short term debt:                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Senior notes:                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|8.375%, face amount of $70, due August 15, 2012                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Other debt                                                                           | 13                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Total short term debt                                                                | 13                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|Long term debt:                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Senior notes of CNAF:                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|5.850%, face amount of $549, due December 15, 2014                                   | 548                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|6.500%, face amount of $350, due August 15, 2016                                     | 348                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|6.950%, face amount of $150, due January 15, 2018                                    | 149                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|7.350%, face amount of $350, due November 15, 2019                                   | 348                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|5.875%, face amount of $500, due August 15, 2020                                     | 496                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|5.750%, face amount of $400, due August 15, 2021                                     | 397                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023                | 241                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036 | 30                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Total long term debt                                                                 | 2,557                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Total debt                                                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Maturity of debt                                                                     | Maturity of Debt(In millions)2013$1320145492015—20163502017—Thereafter1,673Less discount(15)Total$2,570                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
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|(In millions)                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|2013                                                                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|2014                                                                                 | 549                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|2015                                                                                 | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|2016                                                                                 | 350                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|2017                                                                                 | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Thereafter                                                                           | 1,673                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Less discount                                                                        | (15                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Total                                                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

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