| **General**                        | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| **Accounting Policies [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| General                            | GeneralBasis of PresentationThe Condensed 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 92% of the outstanding common stock of CNAF as of March 31, 2024.The accompanying Condensed 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.  Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, including certain financial statement notes, is not required for interim reporting purposes and has been condensed or omitted.  These statements should be read in conjunction with the Consolidated Financial Statements and notes thereto included in CNAF's Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) for the year ended December 31, 2023, including the summary of significant accounting policies in Note A.  The preparation of Condensed 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 Condensed Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting period.  Actual results may differ from those estimates.The interim financial data as of March 31, 2024 and for the three months ended March 31, 2024 and 2023 is unaudited.  However, in the opinion of management, the interim data includes all adjustments, including normal recurring adjustments, necessary for a fair statement of the Company's results for the interim periods in accordance with GAAP.  The results of operations for the interim periods are not necessarily indicative of the results to be expected for the full year.  Accounting Standards Pending AdoptionIn November 2023, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures.  The updated accounting guidance requires expanded reportable segment disclosures, primarily related to significant segment expenses which are regularly provided to the company’s Chief Operating Decision Maker.  The guidance is effective for fiscal years beginning after December 15, 2023, and interim periods within annual periods beginning after December 15, 2024.  Retrospective application is required.  The Company is currently evaluating the effect the updated guidance will have on the Company's financial statement disclosures.In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures.  The updated accounting guidance requires expanded income tax disclosures, including the disaggregation of existing disclosures related to the tax rate reconciliation and income taxes paid.  The guidance is effective for annual periods beginning after December 15, 2024.  Prospective application is required, with retrospective application permitted.  The Company is currently evaluating the effect the updated guidance will have on the Company's financial statement disclosures. |

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| - References
No definition available.
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| - Definition
The entire disclosure for all significant accounting policies of the reporting entity.
+ References
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