| **Summary of Significant Accounting Policies** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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| **Accounting Policies [Abstract]**             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Summary of Significant Accounting Policies     | Summary of Significant Accounting PoliciesBasis of PresentationThe Consolidated Financial Statements include the accounts of CNA Financial Corporation (CNAF) and its subsidiaries.  Collectively, CNAF and its subsidiaries are referred to as CNA or the Company.  Loews Corporation (Loews) owned approximately 89% of the outstanding common stock of CNAF as of December 31, 2017.The accompanying Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP).  Intercompany amounts have been eliminated.  The preparation of Consolidated Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting period.  Actual results may differ from those estimates.Insurance OperationsPremiums: Insurance premiums on property and casualty insurance contracts are recognized in proportion to the underlying risk insured and are principally earned ratably over the duration of the policies.  Premiums on long term care contracts are earned ratably over the policy year in which they are due.  The reserve for unearned premiums represents the portion of premiums written relating to the unexpired terms of coverage.Insurance receivables include balances due currently or in the future, including amounts due from insureds related to losses under high deductible policies, and are presented at unpaid balances, net of an allowance for uncollectible receivables.  Amounts are considered past due based on policy payment terms.  The allowance is determined based on periodic evaluations of aged receivables, management's experience and current economic conditions.  Insurance receivables and any related allowance are written off after collection efforts are exhausted or a negotiated settlement is reached.Property and casualty contracts that are retrospectively rated contain provisions that result in an adjustment to the initial policy premium depending on the contract provisions and loss experience of the insured during the experience period.  For such contracts, the Company estimates the amount of ultimate premiums that the Company may earn upon completion of the experience period and recognizes either an asset or a liability for the difference between the initial policy premium and the estimated ultimate premium.  The Company adjusts such estimated ultimate premium amounts during the course of the experience period based on actual results to date.  The resulting adjustment is recorded as either a reduction of or an increase to the earned premiums for the period.Claim and claim adjustment expense reserves: Claim and claim adjustment expense reserves, except reserves for structured settlements not associated with asbestos and environmental pollution (A&EP), workers' compensation lifetime claims and long term care claims, are not discounted and are based on i) case basis estimates for losses reported on direct business, adjusted in the aggregate for ultimate loss expectations; ii) estimates of incurred but not reported (IBNR) losses; iii) estimates of losses on assumed reinsurance; iv) estimates of future expenses to be incurred in the settlement of claims; v) estimates of salvage and subrogation recoveries and vi) estimates of amounts due from insureds related to losses under high deductible policies.  Management considers current conditions and trends as well as past Company and industry experience in establishing these estimates.  The effects of inflation, which can be significant, are implicitly considered in the reserving process and are part of the recorded reserve balance.  Ceded claim and claim adjustment expense reserves are reported as a component of Reinsurance receivables on the Consolidated Balance Sheets.  Claim and claim adjustment expense reserves are presented net of anticipated amounts due from insureds related to losses under deductible policies of $1.2 billion and $1.2 billion as of December 31, 2017 and 2016.  A significant portion of these amounts are supported by collateral.  The Company has an allowance for uncollectible deductible amounts, which is presented as a component of the allowance for doubtful accounts included in Insurance receivables onthe Consolidated Balance Sheets.Structured settlements have been negotiated for certain property and casualty insurance claims.  Structured settlements are agreements to provide fixed periodic payments to claimants.  The Company's obligations for structured settlements not funded by annuities are included in claim and claim adjustment expense reserves and carried at present values determined using interest rates ranging from to 5.5% to 8.0% as of December 31, 2017 and 2016.  As of December 31, 2017 and 2016, the discounted reserves for unfunded structured settlements were $527 million and $544 million, net of discount of $798 million and $841 million.  For the years ended December 31, 2017, 2016 and 2015, the amount of interest recognized on the discounted reserves of unfunded structured settlements was $41 million, $42 million and $42 million, respectively.  This interest accretion is presented as a component of Insurance claims and policyholders’ benefits on the Consolidated Statements of Operations, but is excluded from the Company’s disclosure of prior year development.Workers' compensation lifetime claim reserves are calculated using mortality assumptions determined through statutory regulation and economic factors.  At December 31, 2017 and 2016, workers' compensation lifetime claim reserves are discounted at a 3.5% interest rate. As of December 31, 2017 and 2016, the discounted reserves for workers’ compensation lifetime claim reserves were $346 millionand $371 million, net of discount of $190 million and $202 million.  For the years ended December 31, 2017, 2016 and 2015 the amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves was $19 million, $17 million and $20 million, respectively.  This interest accretion is presented as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations, but is excluded from the Company's disclosure of prior year development. Long term care claim reserves are calculated using mortality and morbidity assumptions based on Company and industry experience.  Long term care claim reserves are discounted at an interest rate of 6.0% as of December 31, 2017 and interest rates ranging from 4.5% to 6.8% as of December 31, 2016.  As of December 31, 2017 and 2016, such discounted reserves totaled $2.4 billion and $2.2 billion, net of discount of $446 million and $529 million. Future policy benefit reserves: Future policy benefit reserves represent the active life reserves related to the Company's long term care policies and are computed using the net level premium method, which incorporates actuarial assumptions as to morbidity, persistency, inclusive of mortality, discount rate, future premium rate adjustments and expenses.  Expense assumptions primarily relate to claim adjudication.  Actuarial assumptions generally vary by plan, age at issue, policy duration and gender.  The initial assumptions are determined at issuance, including a margin for adverse deviation, and are locked in throughout the life of the contract unless a premium deficiency develops.  If a premium deficiency emerges, the assumptions are unlocked and deferred acquisition costs, if any, and the future policy benefit reserves are adjusted.  The December 31, 2015 gross premium valuation (GPV) indicated a premium deficiency of $296 million.  The indicated premium deficiency necessitated a charge to income that was affected by the write off of the entire long term care deferred acquisition cost asset of $289 million and an increase to active life reserves of $7 million.  The GPV as of December 31, 2017 and 2016 indicated the carried reserves were sufficient; therefore there was no unlocking of assumptions.  Interest rates for long term care active life reserves range from 6.6% to 7.0% as of December 31, 2017 and 2016.Guaranty fund and other insurance-related assessments: Liabilities for guaranty fund and other insurance-related assessments are accrued when an assessment is probable, when it can be reasonably estimated and when the event obligating the entity to pay an imposed or probable assessment has occurred.  Liabilities for guaranty funds and other insurance-related assessments are not discounted and are included as part of Other liabilities on the Consolidated Balance Sheets.  As of December 31, 2017 and 2016, the liability balances were $121 million and $125 million.Reinsurance:  Reinsurance accounting allows for contractual cash flows to be reflected as premiums and losses.  To qualify for reinsurance accounting, reinsurance agreements must include risk transfer.  To meet risk transfer requirements, a reinsurance contract must include both insurance risk, consisting of underwriting and timing risk, and a reasonable possibility of a significant loss for the assuming entity.Reinsurance receivables related to paid losses are presented at unpaid balances.  Reinsurance receivables related to unpaid losses are estimated in a manner consistent with claim and claim adjustment expense reserves or future policy benefit reserves.  Reinsurance receivables are reported net of an allowance for uncollectible amounts on the Consolidated Balance Sheets.  The cost of reinsurance is primarily accounted for over the life of the underlying reinsured policies using assumptions consistent with those used to account for the underlying policies or over the reinsurance contract period.  The ceding of insurance does not discharge the primary liability of the Company.The Company has established an allowance for uncollectible reinsurance receivables which relates to both amounts already billed on ceded paid losses as well as ceded reserves that will be billed when losses are paid in the future.  The allowance for uncollectible reinsurance receivables is estimated on the basis of periodic evaluations of balances due from reinsurers, reinsurer solvency, industry experience and current economic conditions.  Reinsurer financial strength ratings are updated and reviewed on an annual basis or sooner if the Company becomes aware of significant changes related to a reinsurer.  Because billed receivables generally approximate 3% or less of total reinsurance receivables, the age of the reinsurance receivables related to paid losses is not a significant input into the allowance analysis.  Changes in the allowance for uncollectible reinsurance receivables are presented as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations.Amounts are considered past due based on the reinsurance contract terms.  Reinsurance receivables related to paid losses and any related allowance are written off after collection efforts have been exhausted or a negotiated settlement is reached with the reinsurer.  Reinsurance receivables from insolvent insurers related to paid losses are written off when the settlement due from the estate can be reasonably estimated.  At the time reinsurance receivables related to paid losses are written off, any required adjustment to reinsurance receivables related to unpaid losses is recorded as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations.Reinsurance contracts that do not effectively transfer the economic risk of loss on the underlying policies are recorded using the deposit method of accounting, which requires that premium paid or received by the ceding company or assuming company be accounted for as a deposit asset or liability.  The Company had $8 million and $3 million recorded as deposit assets as of December 31, 2017 and 2016, and $4 million and $6 million recorded as deposit liabilities as of December 31, 2017 and 2016.  Income on reinsurance contracts accounted for under the deposit method is recognized using an effective yield based on the anticipated timing of payments and the remaining life of the contract.  When the anticipated timing of payments changes, the effective yield is recalculated to reflect actual payments to date and the estimated timing of future payments.  The deposit asset or liability is adjusted to the amount that would have existed had the new effective yield been applied since the inception of the contract.A loss portfolio transfer is a retroactive reinsurance contract.  If the cumulative claim and allocated claim adjustment expenses ceded under a loss portfolio transfer exceed the consideration paid, the resulting gain from such excess is deferred and amortized into earnings in future periods in proportion to actual recoveries under the loss portfolio transfer.  In any period in which there is a gain position and a revised estimate of claim and allocated claim adjustment expenses, a portion of the deferred gain is cumulatively recognized in earnings as if the revised estimate was available at the inception date of the loss portfolio transfer.Deferred acquisition costs: Deferrable acquisition costs include commissions, premium taxes and certain underwriting and policy issuance costs which are incremental direct costs of successful contract acquisitions.  Acquisition costs related to property and casualty business are deferred and amortized ratably over the period the related premiums are earned.  Deferred acquisition costs are presented net of ceding commissions and other ceded acquisition costs.The Company evaluates deferred acquisition costs for recoverability.  Anticipated investment income is considered in the determination of the recoverability of deferred acquisition costs.  Adjustments, if necessary, are recorded in current period results of operations.  Deferred acquisition costs related to long term care contracts issued prior to January 1, 2004 include costs which vary with and are primarily related to the acquisition of business.  As noted under Future policy benefit reserves, all of the long term care deferred acquisition costs of $289 million were written off as of December 31, 2015 in recognition of a premium deficiency.Investments in life settlement contracts and related revenue recognition:  Prior to 2002, the Company purchased investments in life settlement contracts.  The Company obtained the ownership and beneficiary rights of an underlying life insurance policy through a life settlement contract with the owner of the life insurance contract. The entire portfolio of life settlement contracts, which is included within the Life & Group segment, was determined to be held for sale as of December 31, 2016 as the Company reached an agreement on terms to sell the portfolio.  As such, the Company adjusted the fair value to the estimated sales proceeds less cost to sell.  This resulted in a $10 million loss recognized within Net realized investment gains for the year ended December 31, 2016.  The definitive Purchase and Sale Agreement (PSA) related to the portfolio was executed on March 7, 2017 (sale date).  In connection therewith, the life settlement contracts and related sale proceeds were placed in escrow until the buyer was recognized as the owner and beneficiary of each individual life settlement contract by the life insurance company that issued the policy.  All of the contracts have been released from escrow as of December 31, 2017.  The Company derecognized the released contracts and recorded the consideration, including a note receivable, which is payable over three years and is carried at amortized cost less any valuation allowance.  The note receivable of $46 million is included within Other assets on the December 31, 2017 Consolidated Balance Sheet and interest income is accreted to the principal balance of the note.The fair value of the Company's investments in life settlement contracts was $58 million as of December 31, 2016, and was included in Other assets on the Consolidated Balance Sheets.  The cash receipts and payments related to the life settlement contracts prior to the sale date are included in Cash Flows from operating activities on the Consolidated Statements of Cash Flows.  Cash receipts related to the sale of the life settlement contracts, as well as principal payments on the note receivable, are included in Cash Flows from investing activities.Historically, the Company accounted for its investments in life settlement contracts using the fair value method.  Under the fair value method, each life settlement contract is carried at its fair value at the end of each reporting period.  The change in fair value estimated through the  Company's internal valuation process, life insurance proceeds received and periodic maintenance costs, such as premiums, necessary to keep the underlying policy in force, were recorded in Other revenues on the Consolidated Statements of Operations.The increase in fair value recognized in Other revenues for the years ended December 31, 2016 and 2015 on contracts still held at each respective period-end was $7 million and $1 million.  The gains recognized during the years ended December 31, 2017, 2016 and 2015 on contracts that settled were $3 million, $8 million and $24 million.  InvestmentsThe Company classifies its fixed maturity securities and its equity securities as either available-for-sale or trading, and as such, they are carried at fair value.  The Company's equity holdings substantially consist of non-redeemable preferred stock with characteristics of debt securities, purchased for income generation.  These securities are interest rate sensitive, and typically include stated dividend payment rates and dates and call provisions at the option of the issuer.  Changes in fair value of trading securities are reported within Net investment income on the Consolidated Statements of Operations.  Changes in fair value related to available-for-sale securities are reported as a component of Other comprehensive income.  Losses may be recognized within Net realized investment gains (losses) on the Consolidated Statements of Operations when a decline in value is determined by the Company to be other-than-temporary.The cost of fixed maturity securities classified as available-for-sale is adjusted for amortization of premiums and accretion of discounts, which are included in Net investment income on the Consolidated Statements of Operations.  The amortization of premium and accretion of discount for fixed maturity securities takes into consideration call and maturity dates that produce the lowest yield.  In 2015, the Company changed its accounting principle as previously the amortization of premiums was to maturity.  This change in estimate effected by a change in accounting principle was adopted in the fourth quarter of 2015 and decreased Net investment income and the amortized cost of fixed maturity securities by $39 million in the Consolidated Statement of Operations for the year-ended December 31, 2015 and the Consolidated Balance Sheet as of December 31, 2015.  The $39 million decrease to Net investment income included a $22 million cumulative adjustment relating to prior periods.  The total adjustment decreased Basic and Diluted earnings per share by $0.09 for the year ended December 31, 2015.  For asset-backed securities included in fixed maturity securities, the Company recognizes income using an effective yield based on anticipated prepayments and the estimated economic life of the securities.  When estimates of prepayments change, the effective yield is recalculated to reflect actual payments to date and anticipated future payments.  To the extent that unrealized gains on fixed income securities supporting long term care products and structured settlements not funded by annuities would result in a premium deficiency if those gains were realized, a related increase in Insurance reserves is recorded, net of tax, as a reduction of net unrealized gains through Other comprehensive income (Shadow Adjustments).  Shadow Adjustments, net of tax, increased $397 million and decreased $97 million for the years ended December 31, 2017 and 2016, respectively.  As of December 31, 2017 and 2016, net unrealized gains on investments included in Accumulated other comprehensive income (AOCI) were correspondingly reduced by Shadow Adjustments of $1,411 million and $1,014 million.The Company's carrying value of investments in limited partnerships is its share of the net asset value of each partnership, as determined by the general partner.  Certain partnerships for which results are not available on a timely basis are reported on a lag, primarily three months or less.  Changes in net asset values are accounted for under the equity method and recorded within Net investment income on the Consolidated Statements of Operations.  Mortgage loans are commercial in nature, are carried at unpaid principal balance, net of unamortized fees and any valuation allowance, and are recorded once funded.  Interest income from mortgage loans is recognized on an accrual basis using the effective yield method.  Mortgage loans are considered to be impaired loans when it is probable that contractual principal and interest payments will not be collected.  The Company evaluates loans for impairment on a specific loan basis and identifies loans for evaluation of impairment based on the collection experience of each loan and other credit quality indicators such as debt service coverage ratio and the creditworthiness of the borrower or tenants of credit tenant loan properties.  Accrual of income is generally suspended for mortgage loans that are impaired and collection of principal and interest payments is unlikely.  Mortgage loans are considered past due when full principal or interest payments have not been received according to contractual terms.  As of December 31, 2017 and 2016, there were no loans past due or in non-accrual status, and no valuation allowance was recorded.  Other invested assets include overseas deposits and Federal Home Loan Bank of Chicago (FHLBC) stock.  FHLBC stock is carried at fair value.  Overseas deposits are valued using the net asset value per share (or equivalent) practical expedient.  They are primarily short-term government securities, agency securities and corporate bonds held in trusts that are managed by Lloyd's of London.  These funds are required of Lloyd's syndicates to protect policyholders in overseas markets and may be denominated in local currency.  Short term investments are carried at fair value, with the exception of cash accounts earning interest, which are carried at cost and approximate fair value.  Changes in fair value are reported as a component of Other comprehensive income.Purchases and sales of all securities are recorded on the trade date, except for private placement debt securities, including bank loan participations, which are recorded once funded.  Realized investment gains and losses are determined on the basis of the cost or amortized cost of the specific securities sold.In the normal course of investing activities, the Company enters into relationships with variable interest entities (VIEs), as both an investor in limited partnerships and asset-backed securities issued by third-party VIEs.  The Company is not the primary beneficiary of these VIEs, and therefore does not consolidate them.  The Company determines whether it is the primary beneficiary of a VIE based on a qualitative assessment of the relative power and benefits of the Company and the other participants in the VIE.  The Company’s maximum exposure to loss with respect to these investments is limited to the investment carrying values included in the Company’s Consolidated Balance Sheets and any unfunded commitments.A security is impaired if the fair value of the security is less than its cost adjusted for accretion, amortization and previously recorded OTTI losses, otherwise defined as an unrealized loss.  When a security is impaired, the impairment is evaluated to determine whether it is temporary or other-than-temporary.Significant judgment is required in the determination of whether an OTTI loss has occurred for a security.  The Company follows a consistent and systematic process for determining and recording an OTTI loss, including the evaluation of securities in an unrealized loss position on at least a quarterly basis.The Company’s assessment of whether an OTTI loss has occurred incorporates both quantitative and qualitative information.  Fixed maturity securities that the Company intends to sell, or it more likely than not will be required to sell before recovery of amortized cost, are considered to be other-than-temporarily impaired and the entire difference between the amortized cost basis and fair value of the security is recognized as an OTTI loss in earnings.  The remaining fixed maturity securities in an unrealized loss position are evaluated to determine if a credit loss exists.  The factors considered include (a) the financial condition and near-term and long-term prospects of the issuer, (b) whether the debtor is current on interest and principal payments, (c) credit ratings of the securities and (d) general market conditions and industry or sector specific outlook.  The Company also considers results and analysis of cash flow modeling for asset-backed securities, and when appropriate, other fixed maturity securities.  The focus of the analysis for asset-backed securities is on assessing the sufficiency and quality of underlying collateral and timing of cash flows based on scenario tests.  If the present value of the modeled expected cash flows equals or exceeds the amortized cost of a security, no credit loss is judged to exist and the asset-backed security is deemed to be temporarily impaired.  If the present value of the expected cash flows is significantly less than amortized cost, the security is judged to be other-than-temporarily impaired for credit reasons and that shortfall, referred to as the credit component, is recognized as an OTTI loss in earnings.  The difference between the adjusted amortized cost basis and fair value, referred to as the non-credit component, is recognized as OTTI in Other comprehensive income.  In subsequent reporting periods, a change in intent to sell or further credit impairment on a security whose fair value has not deteriorated will cause the non-credit component originally recorded as OTTI in Other comprehensive income to be recognized as an OTTI loss in earnings.  The Company performs the discounted cash flow analysis using stressed scenarios to determine future expectations regarding recoverability.  Significant assumptions enter into these cash flow projections including delinquency rates, probable risk of default, loss severity upon a default, over collateralization and interest coverage triggers and credit support from lower level tranches.The Company applies the same impairment model as described above for the majority of its non-redeemable preferred stock securities on the basis that these securities possess characteristics similar to debt securities.  For all other equity securities, in determining whether the security is other-than-temporarily impaired, the Company considers a number of factors including, but not limited to: (a) the length of time and the extent to which the fair value has been less than amortized cost, (b) the financial condition and near term prospects of the issuer, (c) the intent and ability of the Company to retain its investment for a period of time sufficient to allow for an anticipated recovery in value and (d) general market conditions and industry or sector specific outlook.Income TaxesThe Company and its eligible subsidiaries (CNA Tax Group) are included in the consolidated federal income tax return of Loews and its eligible subsidiaries.  The Company accounts for income taxes under the asset and liability method.  Under the asset and liability method, deferred income taxes are recognized for temporary differences between the financial statement and tax return bases of assets and liabilities, based on enacted tax rates and other provisions of the tax law.  The effect of a change in tax laws or rates on deferred tax assets and liabilities is recognized in income in the period in which such change is enacted.  Future tax benefits are recognized to the extent that realization of such benefits is more likely than not, and a valuation allowance is established for any portion of a deferred tax asset that management believes will not be realized. Pension and Postretirement BenefitsThe Company recognizes the overfunded or underfunded status of its defined benefit plans in Other assets or Other liabilities on the Consolidated Balance Sheets.  Changes in funded status related to prior service costs and credits and actuarial gains and losses are recognized in the year in which the changes occur through Other comprehensive income.  Annual service cost, interest cost, expected return on plan assets, amortization of prior service costs and credits and amortization of actuarial gains and losses are recognized in the Consolidated Statements of Operations.  The vested benefit obligation for the CNA Retirement Plan is determined based on eligible compensation and accrued service for previously entitled employees.  Effective June 30, 2015, future benefit accruals under the CNA Retirement Plan were eliminated and the benefit obligations were frozen.Stock-Based CompensationThe Company records compensation expense using the fair value method for all awards it grants, modifies or cancels primarily on a straight-line basis over the requisite service period, generally three to four years.Foreign CurrencyForeign currency translation gains and losses are reflected in Stockholders' equity as a component of AOCI.  The Company's foreign subsidiaries' balance sheet accounts are translated at the exchange rates in effect at each reporting date and income statement accounts are either translated at the exchange rate on the date of the transaction or at the average exchange rates.  Foreign currency transaction gains (losses) of $27 million, $(9) million and $(11) million were included in determining Net income (loss) for the years ended December 31, 2017, 2016 and 2015.Property and EquipmentProperty and equipment are carried at cost less accumulated depreciation.  Depreciation is based on the estimated useful lives of the various classes of property and equipment and is determined principally on the straight-line method.  Furniture and fixtures are depreciated over seven years.  Office equipment is depreciated over five years.  The estimated lives for data processing equipment and software generally range from three to five years, but can be as long as ten years.  Leasehold improvements are depreciated over the corresponding lease terms not to exceed the underlying asset life.  The Company's previously owned building and related capital improvements were depreciated over periods not to exceed fifty years. GoodwillGoodwill represents the excess of purchase price over the fair value of the net assets of acquired entities and businesses.  Goodwill in the International segment may change from period to period as a result of foreign currency translation.  Goodwill is tested for impairment annually or when certain triggering events require such tests. As a result of reviews completed for the year ended December 31, 2017, the Company determined that the estimated fair value of the reporting units were in excess of their carrying value including Goodwill.  Changes in future periods in assumptions about the level of economic capital, business growth, earnings projections or the weighted average cost of capital could result in a goodwill impairment.Other Intangible AssetsOther intangible assets are reported within Other assets.Finite-lived intangible assets are amortized over their estimated useful lives.  Indefinite-lived other intangible assets are tested for impairment annually or when certain triggering events require such tests.  Earnings (Loss) Per Share DataEarnings (loss) per share is based on weighted average number of outstanding common shares.  Basic earnings (loss) per share excludes the impact of dilutive securities and is computed by dividing Net income (loss) by the weighted average number of common shares outstanding for the period.  Diluted earnings (loss) per share reflects the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock.For the years ended December 31, 2017, 2016 and 2015, approximately 988 thousand, 678 thousand and 534 thousand potential shares attributable to exercises or conversions into common stock under stock-based employee compensation plans were included in the calculation of diluted earnings per share.  For those same periods, less than 1 thousand, 102 thousand and 106 thousand potential shares attributable to exercises or conversions into common stock under stock-based employee compensation plans were not included in the calculation of diluted earnings per share because the effect would have been antidilutive.Supplementary Cash Flow InformationCash payments made for interest were $155 million, $157 million and $152 million for the years ended December 31, 2017, 2016 and 2015.  Cash payments made for income taxes were $152 million, $170 million and $310 million for the years ended December 31, 2017, 2016 and 2015. Recently Adopted Accounting Standards Updates (ASU) In March 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvementsto Employee Share-Based Payment Accounting.  The updated accounting guidance simplifies the accounting for share-based payment award transactions, including income tax consequences, classification of awards as either equity or liabilities and classification on the statement of cash flows.  As of January 1, 2017, the Company adopted the updated accounting guidanceand began recognizing excess tax benefits or deficiencies on vesting or settlement of awards as an income tax benefit or expense within net income, instead of additional paid-in capital as required under previous guidance.  The related cash flows are now classified within operating activities.  As a result of this change, excess tax benefits are no longer included in assumed proceeds under the treasury stock method of calculating earnings per share.  The impact of the accounting change resulted in a decrease of $6 million to Income tax expense for the year ended December 31, 2017.  Accounting Standards Pending AdoptionIn May 2014, the FASB issued ASU No. 2014-09, Revenue Recognition (Topic 606): Revenue from Contracts with Customers.  The standard excludes from its scope the accounting for insurance contracts, financial instruments, and certain other agreements that are governed under other GAAP guidance. The updated guidance requires an entity to recognize revenue as performance obligations are met, in an amount that reflects the consideration the entity is entitled to receive for the transfer of the promised goods or services. The standard is effective for interim and annual reporting periods beginning after December 15, 2017 and may be applied retrospectively or through a cumulative effect adjustment to retained earnings at the date of adoption.  The Company plans to adopt the new revenue guidance effective January 1, 2018 using the modified retrospective approach.  The Company expects that revenue on warranty products and services will be recognized more slowly under the new guidance than under the current revenue recognition pattern.  At adoption, the Company anticipates a cumulative effect adjustment that will decrease Retained earnings by approximately $65 million.  Additionally, Other revenues and Other operating expenses on the Company's Consolidated Statements of Operations will increase for those contracts for which the Company has concluded it is a principal, as the retail sellers' mark-up will now be reflected as revenue and commission expense.  The estimated annual gross-up of Other revenues and Other operating expenses will be approximately $500 million.  The related gross-up effect on the Consolidated Balance Sheet at adoption will be an increase of Other assets and Other liabilities by approximately $1.7 billion.  Based on the Company’s assessment, the impact of adoption of the new guidance will not be material to the Company’s results of operations or financial position.In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.  The updated accounting guidance requires changes to the reporting model for financial instruments.  The guidance is effective for interim and annual periods beginning after December 15, 2017.  The Company expects the primary change to be the requirement for equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income.  The Company will recognize a cumulative effect adjustment to Retained earnings and AOCI for the net of tax amount of unrealized investment gains and losses related to available-for-sale equity securities at the date of adoption.  At adoption, the Company estimates this new guidance will result in an after-tax increase to Retained earnings and a decrease to AOCI of $28 million.  Subsequent to adoption, changes in the fair value of equity securities will be reported as Net realized investment gains (losses) in the Company's Consolidated Statement of Operations, which will introduce additional volatility in the Company's results of operations.In March 2017, the FASB issued ASU No. 2017-07, Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The updated accounting guidance requires changes to the presentation of the components of net periodic benefit cost on the income statement by requiring service cost to be presented with other employee compensation costs and other components of net periodic pension cost to be presented outside of any subtotal of operating income.  The ASU also stipulates that only the service cost component of net benefit cost is eligible for capitalization. The guidance is effective for interim and annual periods beginning after December 15, 2017.  The guidance is to be applied on a prospective basis for capitalization of service costs and on a retrospective basis for the presentation of the service cost and other components of net periodic benefit costs in the Company's Consolidated Statements of Operations or in its disclosures. The Company will adopt the updated guidance effective January 1, 2018. The Company plans to expand the related footnote disclosure to show the amount of service cost and non-service cost components of net periodic benefit cost and the line items in the Consolidated Statements of Operations in which such amounts are reported. The adoption impact from the change is not expected to be material to the Company’s results of operations or financial position.In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842): Accounting for Leases.  The updated accounting guidance requires lessees to recognize on the balance sheet the assets and liabilities for the rights and obligations created by all leases, including those historically accounted for as operating leases.  The guidance is effective for interim and annual periods beginning after December 15, 2018.  The Company is currently evaluating the effect the updated guidance will have on the Company's financial statements.  It is expected that assets and liabilities will increase based on the present value of remaining lease payments for leases in place at the adoption date; however, this is not expected to be material to the Company's results of operations or financial position.In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurementof Credit Losses on Financial Instruments.  The updated accounting guidance requires changes to the recognition of credit losses on financial instruments not accounted for at fair value through net income.  The guidance is effective for interim and annual periods beginning after December 15, 2019.  The Company is currently evaluating the effect the guidance will have on the Company's financial statements, but expects the primary changes to be the use of the expected credit loss model for its mortgage loan portfolio and reinsurance receivables and the presentation of credit losses within the available-for-sale fixed maturities portfolio through an allowance method rather than as a direct write-down.  The expected credit loss model will require a financial asset to be presented at the net amount expected to be collected.  The allowance method for available-for-sale debt securities will allow the Company to record reversals of credit losses if the estimate of credit losses declines. |

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| - Definition
The entire disclosure for all significant accounting policies of the reporting entity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -URI http://asc.fasb.org/topic&trid=2122369
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