|**Debt (Tables)**                                                     | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
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|**Debt Disclosure [Abstract]**                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Schedule of debt instruments                                          | Debt is composed of the following long-term obligations.December 31(In millions)20232022Short-term debt:Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023$—$243Senior notes of CNAF:3.950%, face amount of $550, due May 15, 2024550—Total short-term debt550243Long-term debt:Senior notes of CNAF:3.950%, face amount of $550, due May 15, 2024—5494.500%, face amount of $500, due March 1, 20265004993.450%, face amount of $500, due August 15, 20274984973.900%, face amount of $500, due May 1, 20294974972.050%, face amount of $500, due August 15, 20304964965.500%, face amount of $500, due June 15, 2033490—Total long-term debt2,4812,538Total debt$3,031$2,781
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|December 31                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|(In millions)                                                         | 2023                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Short-term debt:                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Senior notes of CNAF:                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|3.950%, face amount of $550, due May 15, 2024                         | 550                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total short-term debt                                                 | 550                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Long-term debt:                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Senior notes of CNAF:                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|3.950%, face amount of $550, due May 15, 2024                         | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|4.500%, face amount of $500, due March 1, 2026                        | 500                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|3.450%, face amount of $500, due August 15, 2027                      | 498                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|3.900%, face amount of $500, due May 1, 2029                          | 497                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|2.050%, face amount of $500, due August 15, 2030                      | 496                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|5.500%, face amount of $500, due June 15, 2033                        | 490                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total long-term debt                                                  | 2,481                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Total debt                                                            | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Maturity of debt                                                      | The combined aggregate maturities for debt as of December 31, 2023 are presented in the following table.(In millions)2024$5502025—202650020275002028—Thereafter1,500Less: discount(19)Total$3,031                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|(In millions)                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|2024                                                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|2025                                                                  | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|2026                                                                  | 500                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|2027                                                                  | 500                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|2028                                                                  | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Thereafter                                                            | 1,500                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Less: discount                                                        | (19)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total                                                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

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