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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 89.6% of the outstanding common stock of CNAF as of March 31, 2022.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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| Accounting Standards Pending Adoption | Accounting Standards Pending AdoptionIn August 2018, the FASB issued ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts.  The updated accounting guidance requires changes to the measurement and disclosure of long-duration contracts.  For the Company, this includes the Long term care and fully-ceded single premium immediate annuity business.  Entities will be required to review, and update if there is a change, cash flow assumptions (including morbidity and persistency) at least annually, and to update discount rate assumptions quarterly using an upper-medium grade fixed-income instrument yield.  The effect of changes in cash flow assumptions will be recorded in the Company's results of operations and the effect of changes in discount rate assumptions will be recorded in Other comprehensive income.  The guidance is effective for interim and annual periods beginning after December 15, 2022, with early adoption permitted, and may be applied using either a modified retrospective transition method or a full retrospective transition method.  Financial statements for prior periods presented shall be adjusted to reflect the effects of applying the new accounting guidance.The Company will adopt the new guidance effective January 1, 2023, using the modified retrospective method applied as of the transition date of January 1, 2021.  The Company will use a published spot rate curve constructed from A+, A and A- rated U.S. dollar denominated corporate bonds matched to the duration of the corresponding insurance liabilities, to calculate discount rates.  The Company will group its long-duration contracts into calendar year cohorts based on the contract issue date and product line.  Long term care contracts will be grouped separately from the Company’s fully-ceded single premium immediate annuity contracts. The most significant impact at the transition date will be the effect of updating the discount rate assumption to reflect an upper-medium grade fixed-income instrument yield, which will be partially offset by the de-recognition of Shadow Adjustments associated with long-duration contracts.  The Company expects the net impact of these changes will be a $2.2 billion - $2.5 billion decrease in Accumulated other comprehensive income as of the transition date of January 1, 2021.  There is a minimal transition impact expected to retained earnings. The requirement to review, and update if there is a change, cash flow assumptions at least annually is expected to change the pattern of earnings being recognized.  Adoption will also significantly expand the Company’s disclosures, and will impact systems, processes, and controls.  While the requirements of the new guidance represent a material change from existing GAAP, the new guidance will not impact capital and surplus under statutory accounting practices, cash flows, or the underlying economics of the business.The Company continues to make progress in connection with these matters and is in process of refining key accounting policy decisions, technology solutions and updates to internal controls associated with adoption of the new guidance.  These in-progress activities include modifications of actuarial valuation systems, data sourcing, analytical procedures and reporting processes. |
| Earnings (Loss) Per Share             | Earnings (Loss) Per ShareEarnings (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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |

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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 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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| - Definition
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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