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| **Accounting Policies [Abstract]**    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Basis of Presentation                 | The 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 September 30, 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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Use of Estimates                      | 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 September 30, 2024 and for the three and nine months ended September 30, 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 Adoption | In 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 (CODM).  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 its financial statement disclosures and expects to disclose additional quantitative and qualitative information related to segment expenses regularly provided to the CODM that are included in the Company's measure of segment profit or loss, which is core income (loss). 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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Earnings (Loss) Per Share Data        | Earnings (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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Future policy benefits reserves       | Future Policy Benefits ReservesFuture policy benefits reserves are associated with the Company's run-off long-term care business, included in the Life & Group segment, and relate to policyholders that are currently receiving benefits, including claims that have been incurred but are not yet reported, as well as policyholders that are not yet receiving benefits.  Future policy benefits reserves are comprised of the liability for future policyholder benefits (LFPB) which is reflected as Insurance reserves: Future policy benefits on the Condensed Consolidated Balance Sheet.The determination of Future policy benefits reserves requires management to make estimates and assumptions about expected policyholder experience over the remaining life of the policy.  Since policies may be in force for several decades, these assumptions are subject to significant estimation risk.  As a result of this variability, the Company’s future policy benefits reserves may be subject to material increases if actual experience develops adversely to the Company’s expectations. Annually in the third quarter, actuarial analysis is performed on policyholder morbidity, persistency, premium rate increases and expense experience.  This analysis, combined with judgment, informs the setting of updated cash flow assumptions used to estimate the LFPB.  Actuarial analysis includes predictive modeling, actual to expected experience comparisons and trend analysis.  Applicable industry research is also considered.  The cash flow assumption updates for the third quarter of 2024 resulted in a $15 million pretax increase in the LFPB.  Included in the assumption updates was a favorable impact from outperformance on premium rate assumptions and unfavorable impact from higher cost of care inflation.  The cash flow assumption updates for the third quarter 2023 resulted in an $8 million pretax increase to the LFPB.  Persistency updates were unfavorable due to revisions to lapse rates.   Morbidity updates were favorable driven by claim severity assumption updates, and there was a favorable impact from outperformance on premium rate assumptions.See Note A to the Consolidated Financial Statements within CNAF's Annual Report on Form 10-K for the year ended December 31, 2023 for further information on the long-term care reserving process. |

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| - Definition
Disclosure of accounting policy regarding (1) the principles it follows in consolidating or combining the separate financial statements, including the principles followed in determining the inclusion or exclusion of subsidiaries or other entities in the consolidated or combined financial statements and (2) its treatment of interests (for example, common stock, a partnership interest or other means of exerting influence) in other entities, for example consolidation or use of the equity or cost methods of accounting. The accounting policy may also address the accounting treatment for intercompany accounts and transactions, noncontrolling interest, and the income statement treatment in consolidation for issuances of stock by a subsidiary.
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| - Definition
Disclosure of accounting policy for computing basic and diluted earnings or loss per share for each class of common stock and participating security. Addresses all significant policy factors, including any antidilutive items that have been excluded from the computation and takes into account stock dividends, splits and reverse splits that occur after the balance sheet date of the latest reporting period but before the issuance of the financial statements.
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Disclosure of accounting policy for liability for future benefit to be paid to or on behalf of policyholder. Includes, but is not limited to, input, judgment, assumption, and method used in measuring liability and change in input, judgment, and assumption.
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| - Definition
Disclosure of accounting policy pertaining to new accounting pronouncements that may impact the entity's financial reporting. Includes, but is not limited to, quantification of the expected or actual impact.
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Disclosure of accounting policy for the use of estimates in the preparation of financial statements in conformity with generally accepted accounting principles.
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