| **General (Policies)**                                                                         | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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| **Accounting Policies [Abstract]**                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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| Basis of Presentation                                                                          | Basis 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 89.6% of the outstanding common stock of CNAF as of September 30, 2020. 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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            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| Recently Adopted Accounting Standards Updates (ASUs) and Accounting Standards Pending Adoption | Recently Adopted Accounting Standards Updates (ASU) ASU 2016-13: In June 2016 the Financial Accounting Standards Board (FASB) issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of 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 the Company’s results of operations.  For financial assets measured at cost, the expected credit loss model requires immediate recognition of estimated credit losses over the life of the asset and presentation of the asset at the net amount expected to be collected.  This new guidance applies to mortgage loan investments, reinsurance and insurance receivables and other financing receivables.  For available-for-sale fixed maturity securities carried at fair value, estimated credit losses will continue to be measured at the present value of expected cash flows, however, the other than temporary impairment (OTTI) concept has been eliminated.  Under the previous guidance, estimated credit impairments resulted in a write-down of amortized cost.  Under the new guidance, estimated credit losses are recognized through an allowance and reversals of the allowance are permitted if the estimate of credit losses declines.  For available-for-sale fixed maturity securities where the Company has an intent to sell, impairment will continue to result in a write-down of amortized cost. On January 1, 2020, the Company adopted the updated guidance using a modified retrospective method with a cumulative effect adjustment recorded to beginning Retained earnings.  Prior period amounts have not been adjusted and continue to be reported in accordance with the previous accounting guidance.  A prospective transition approach is required for available-for-sale fixed maturity securities that were purchased with credit deterioration (PCD assets) or have recognized an OTTI write-down prior to the effective date.  The cumulative effect of the accounting change resulted in a $5 million decrease in Retained earnings, with a corresponding $7 million allowance for credit losses recorded for Mortgage loans partially offset by a $2 million tax impact.The allowance for uncollectible insurance and reinsurance receivables was unchanged as a result of adopting the new guidance.  At adoption, an allowance for credit losses of $6 million was established for available-for-sale fixed maturity securities that were PCD assets, with a corresponding increase to amortized cost, resulting in no adjustment to the carrying value of the securities.  Below is a summary of the significant accounting policies impacted by the adoption of ASU 2016-13.The allowance for credit losses is a valuation account that is reported as a reduction of a financial asset’s cost basis and is measured on a pool basis when similar risk characteristics exist.  Management estimates the allowance using relevant available information from both internal and external sources.  Historical credit loss experience provides the basis for the estimation of expected credit losses and adjustments may be made to reflect current conditions and reasonable and supportable forecasts.  Adjustments to historical loss information are made for any additional factors that come to the Company’s attention.  This could include significant shifts in counterparty financial strength ratings, aging of past due receivables, amounts sent to collection agencies, or other underlying portfolio changes.  Amounts are considered past due when payments have not been received according to contractual terms.  The Company also considers current and forecast economic conditions, using a variety of economic metrics and forecast indices.  The sensitivity of expected credit losses relative to changes to these forecast economic conditions can vary by financial asset class.  The Company considers a reasonable and supportable forecast period to be up to 24 months from the balance sheet date. After the forecast period, the Company reverts to historical credit experience.  The Company uses collateral arrangements such as letters of credit and amounts held in beneficiary trusts to mitigate credit risk, which are considered in the estimate of net amount expected to be collected.The Company has made a policy election to present accrued interest balances separately from the amortized cost basis of assets and has elected the practical expedient to exclude the accrued interest from the tabular disclosures for mortgage loans and available-for-sale securities.  The Company has elected not to estimate an allowance for credit losses on accrued interest receivable.  The accrual of interest income is discontinued and the asset is placed on nonaccrual status within 90 days of the interest becoming delinquent.  Interest accrued but not received for assets on nonaccrual status is reversed through investment income.  Interest received for assets that are on nonaccrual status is recognized as payment is received.  The asset is returned to accrual status when the principal and interest amounts contractually due are brought current and future payments are expected.  Interest receivable is presented as a component of accrued investment income on the Condensed Consolidated Balance Sheet.See Note C and Note K to the Condensed Consolidated Financial Statements for additional information regarding credit losses. 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.  The guidance requires entities to annually update cash flow assumptions, including morbidity and persistency, and 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.  This guidance is effective for interim and annual periods beginning after December 15, 2021, however the FASB has approved a one year deferral of the effective date.  Early adoption is permitted.  The Company may elect to apply the guidance using either a modified retrospective transition method or a full retrospective transition method.  The guidance requires restatement of prior periods presented.  The Company plans to adopt using the modified retrospective transition method and is currently evaluating the effect the updated guidance will have on its financial statements, including the increased disclosure requirements.  The annual updating of cash flow assumptions is expected to increase income statement volatility.  While the requirements of the new guidance represent a material change from existing GAAP, the underlying economics of the business and related cash flows will be unchanged. |
| Earnings per share                                                                             | Earnings (Loss) Per Share 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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 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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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| - 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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No definition available.
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