| **General (Policies)**                              | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
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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 90% of the outstanding common stock of CNAF as of June 30, 2023.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 Updates (ASU) Pending Adoption | Recently Adopted Accounting Standards Updates (ASU)ASU 2018-12: In August 2018, the Financial Accounting Standards Board (FASB) issued ASU 2018-12, which requires changes to the measurement and disclosure of long-duration contracts.  Entities are required to review, and update if there is a change, cash flow assumptions (including morbidity and persistency) used to measure the liability for future policyholder benefits (LFPB) at least annually.  The LFPB must also be updated for actual experience at least annually.  The LFPB is reflected as Insurance reserves: Future policy benefits on the Condensed Consolidated Balance Sheet.  The discount rate assumption used to measure the LFPB must be updated quarterly using an upper-medium grade (low credit risk) fixed-income instrument yield, commonly interpreted as a single-A rate.  The effect of changes in cash flow assumptions and actual variances from expected experience are recorded in the Company's results of operations within Insurance claims and policyholders’ benefits.  The effect of changes in discount rate assumptions are recorded in Other comprehensive income (loss).  In contrast, under legacy accounting guidance, cash flow and discount rate assumptions were locked-in unless a premium deficiency emerged.  The discount rate assumption under legacy accounting guidance was determined using the Company’s internal investment portfolio yield, which was generally higher than a single-A yield.The new guidance eliminates the need to hold shadow reserves associated with the Company’s long term care reserves.  Under legacy accounting guidance, to the extent that unrealized gains on fixed maturity securities supporting long term care reserves would have resulted in a premium deficiency if realized, a related increase to Insurance reserves was recorded, net of tax, as a reduction of net unrealized gains (losses), through Other comprehensive income (loss) (shadow reserves). The unit of account is the level at which reserves are measured.  Under the new guidance, the unit of account used to measure the LFPB is the cohort.  Cohorts are comprised of insurance contracts issued no more than one year apart, and must be further disaggregated according to policy benefit and insurance risk characteristics.  Under legacy accounting guidance, the LFPB was generally measured at the individual policy level.Under the new guidance, the Net Premium Ratio (NPR) is capped at 100%.  To the extent that NPR would otherwise exceed 100%, the LFPB is increased and a loss is recognized immediately in the Company’s results of operations.  The NPR cap is applied at the cohort level each quarter when NPR is updated.  In contrast, under legacy accounting guidance,  premium deficiency testing was performed annually at the product level.  See Note F to the Condensed Consolidated Financial Statements for further explanation of the NPR and LFPB calculations. The Company adopted the new guidance effective January 1, 2023, using the modified retrospective method applied as of the transition date of January 1, 2021.  The Company's run-off long term care business is in scope of the new guidance.  All prior periods presented in the financial statements have been adjusted to reflect application of the new guidance.  The Company’s original locked in discount rate, utilized for purposes of calculating the NPR under the new guidance, was based on the discount rate assumption used to calculate the LFPB immediately prior to the transition date.  While the requirements of the new guidance represent a material change from legacy accounting, the new guidance does not impact capital and surplus under statutory accounting practices, cash flows or the underlying economics of the business.In December 2022, the FASB issued ASU 2022-05, Financial Services-Insurance (Topic 944): Transition for Sold Contracts (ASU 2022-05).  This guidance permits companies to make an election to exclude from the scope of ASU 2018-12 any insurance contracts that have been de-recognized prior to the effective date of ASU 2018-12, assuming that the company has no significant continuing involvement with the de-recognized contracts.  In the fourth quarter of 2022, the Company novated its block of legacy annuity business, which was fully-ceded prior to novation.  The Company has elected the ASU 2022-05 transition relief, and has excluded the novated legacy annuity business from the scope of ASU 2018-12.                                                                                                                                                                                                                                                                   |
| 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 Benefit Reserves                      | 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. The LFPB is computed using the net level premium method, which incorporates cash flow assumptions and discount rate assumptions.  Under the net level premium method, the LFPB is equal to the present value of future benefits and claim settlement expenses less the present value of future net premiums.  Net premiums are equal to gross premiums multiplied by the NPR.  The NPR is generally the ratio of the present value of benefits and expense payments to the present value of gross premiums, expected over the lifetime of the policy.  As a result of the modified retrospective adoption of ASU 2018-12, the Company’s NPR calculation incorporates the original locked in discount rate and the reserve balance as of the transition date of January 1, 2021.   The key cash flow assumptions used to estimate the LFPB are morbidity, persistency (inclusive of mortality), anticipated future premium rate increases and expenses.  Morbidity is the frequency and severity of injury, illness, sickness and diseases contracted.  Persistency is the percentage of policies remaining in force and can be affected by policy lapses, benefit reductions and death.  Future premium rate increases are generally subject to regulatory approval, and therefore the exact timing and size of the approved rate increases are unknown.  Expense assumptions relate to claim adjudication.  The Company has not elected the practical expedient that allows locking in the expense assumption.  The discount rate is determined using the upper-medium grade fixed  income instrument yield curve.The Company has elected to update the NPR and the LFPB for actual experience on a quarterly basis.  A quarterly assessment is also made as to whether evidence suggests that cash flow assumptions should be updated.  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.  Quarterly, to derive the upper-medium grade fixed income instrument yield discount rate assumption, the Company uses a published spot rate curve constructed from single-A rated U.S. dollar denominated corporate bonds.  The Company uses linear interpolation to determine yield assumptions for tenors that fall between points for which observable rates are available.  For cash flows that are projected to occur beyond the tenor for which market-observable rates are available, the Company applies judgment to estimate a normative rate which the Company grades to over 10 years.Quarterly, the updated NPR is used to derive an updated LFPB as of the beginning of the current quarter measured at the original locked in discount rate.  The updated LFPB is then compared to the existing carrying amount of the liability as of the same date (measured at the original locked in discount rate) to determine the re-measurement gain (loss), which is presented parenthetically within the Insurance claims and policyholders’ benefits line on the Condensed Consolidated Statements of Operations.Insurance contracts are grouped into cohorts according to issue year.  Contracts assumed through reinsurance are generally included within the same cohorts as contracts issued directly by the Company, according to issue year.  The issue year for assumed contracts is defined according to the date that the Company’s assumption of insurance risk incepted.  For assumed contracts that were reinsured concurrently with the issuance of the underlying direct contract, issue year is defined as the year that the underlying policy was issued.  For contracts that were already in-force when assumed by the Company, issue year is defined as the year in which the reinsurance agreement incepted.  For group long term care business, issue year is defined as the year the individual insurance certificate was issued.  Long term care is the Company's only long-duration product line, therefore, cohorts are not further disaggregated by product. |

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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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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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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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