|**General (Policies)**                                                                  | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           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|**Accounting Policies [Abstract]**                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Consolidation                                                                           | 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 approximately89%of the outstanding common stock of CNAF as ofSeptember 30, 2019.The accompanying Condensed Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP).  Intercompany amounts have been eliminated.  Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, including certain financial statement notes, is not required for interim reporting purposes and has been condensed or omitted.  These statements should be read in conjunction with the Consolidated Financial Statements and notes thereto included in CNAF's Annual Report on Form 10-K filed with the Securities and Exchange Commission for the year endedDecember 31, 2018, including the summary of significant accounting policies in NoteA.  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 ofSeptember 30, 2019and for thethree and nine months ended September 30, 2019and2018is 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.  The results of operations for the interim periods are not necessarily indicative of the results to be expected for the full year.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Recently Adopted Accounting Standards Updates and Accounting Standards Pending Adoption | Recently Adopted Accounting Standards Updates (ASU)ASU 2016-02:In February 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-02,Leases (Topic 842): Accounting for Leases.The updated accounting guidance requires lessees to recognize on the balance sheet assets and liabilities for the rights and obligations created by the majority of leases, including those historically accounted for as operating leases.  On January 1, 2019, the Company adopted the updated guidance using a modified retrospective method.  Prior period amounts have not been adjusted and continue to be reported in accordance with the previous accounting guidance.  The Company utilized the package of practical expedients allowing the Company to not reassess whether a contract is or contains a lease, lease classification and initial direct costs.  The Company also utilized the practical expedient to not separate lease and non-lease components for all leases.Adoption of the updated guidance resulted in the following changes to the Condensed Consolidated Balance Sheet on January 1, 2019:(In millions)Balance as of December 31, 2018Adjustments Due to Adoption of Topic 842Balance as of January 1, 2019Property and equipment at cost (less accumulated depreciation)$324$2$326Other assets1,2082371,445Other liabilities3,0892393,328As ofJanuary 1, 2019, operating lease right-of-use (ROU) assets, included within Other assets, were reduced by accrued rent and lease incentives of$75 millionpreviously classified as Other liabilities.  The updated guidance did not impact the Condensed Consolidated Statements of Operations.  See NoteKto the Condensed Consolidated Financial Statements for additional information regarding leases.Accounting Standards Pending AdoptionIn June 2016, the 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 net income.  The guidance is effective for interim and annual periods beginning after December 15, 2019.  The guidance will be applied using a modified retrospective approach with the cumulative effect recognized as an adjustment to retained earnings.  A prospective transition approach is required for debt securities that have recognized an other-than-temporary impairment prior to the effective date.  The Company is currently evaluating the effect the guidance will have on the Company's financial statements, but does not expect the impact to be material.  The primary changes will be the use of the expected credit loss model for mortgage loans, reinsurance and insurance receivables and other financing receivables and the use of the allowance method rather than the write-down method for credit losses within the available-for-sale fixed maturities portfolio.  The expected credit loss model will require a financial asset to be presented at the net amount expected to be collected.  The allowance method for available-for-sale debt securities will allow the Company to record reversals of credit losses if the estimate of credit losses declines.The Company is currently in the process of evaluating existing impairment methodology, developing models to comply with the new guidance and accumulating all of the necessary internal and external information required to measure credit losses under the expected credit loss model.  The Company is implementing changes to the information systems to assist with the accounting, including the recording of the allowance.  The Company is also evaluating additional changes to processes to meet the reporting and disclosure requirements of the new guidance.In 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 Net income 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, 2020; however the FASB has approved a one year deferral of the effective date.  The guidance requires restatement of prior periods presented.  Early adoption is permitted.  The Company is currently evaluating the method and timing of adoption and the effect the updated guidance will have on its financial statements.  The annual updating of cash flow assumptions is expected to increase income statement volatility.  The quarterly change in discount rate is expected to increase volatility in the Company’s stockholders' equity, but that will be somewhat mitigated because Shadow Adjustments are eliminated under the new guidance.  While the requirements of the new guidance represent a material change from existing GAAP, the underlying economics of the business and related cash flows are unchanged.
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|(In millions)                                                                           | Balance as of December 31, 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Property and equipment at cost (less accumulated depreciation)                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Other assets                                                                            | 1,208                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Other liabilities                                                                       | 3,089                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         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