| **General**                                                                                                   | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         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| **Organization, Consolidation and Presentation of Financial Statements [Abstract]**                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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| Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] | Note A. GeneralBasis 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 90% of the outstanding common stock of CNAF as of September 30, 2016.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, including certain financial statement notes, prepared in accordance with GAAP, 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 ended December 31, 2015, including the summary of significant accounting policies in Note A.  The preparation of Condensed Consolidated Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the Condensed Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting period.  Actual results may differ from those estimates.The interim financial data as of September 30, 2016 and for the three and nine months endedSeptember 30, 2016 and 2015 is unaudited.  However, in the opinion of management, the interim data includes all adjustments, including normal recurring adjustments, necessary for a fair statement of the Company's results for the interim periods.  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 (ASU)In April 2015, the Financial Accounting Standards Board (FASB) issued ASU No. 2015-03, Interest-Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs.  The updated accounting guidance requires debt issuance costs to be presented as a deduction from the corresponding debt liability instead of the historical presentation as an unamortized debt issuance asset.  As of January 1, 2016, the Company adopted the updated accounting guidance retrospectively.  The Company adjusted its previously reported financial information included herein to reflect the change in accounting guidance for debt issuance costs.  The impacts of adopting the new accounting standard on the Company’s Consolidated Balance Sheet as of December 31, 2015, were a decrease in Other assets and a decrease in Long term debt of $2 million.   In May 2015, the FASB issued ASU No. 2015-07, Fair Value Measurement (Topic 820): Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent).  The updated accounting guidance removes the requirement to categorize assets measured at fair value utilizing the net asset value per share (or equivalent) practical expedient within the fair value hierarchy.  As of January 1, 2016, the Company adopted the updated accounting guidance retrospectively.  The Company adjusted its previously reported financial information included herein to reflect the change in accounting guidance for assets measured using the net asset value.  The impact of adopting the new accounting standard resulted in excluding overseas deposits of $30 million and $27 million from the fair value level disclosure as of September 30, 2016 and December 31, 2015.Accounting Standards Pending Adoption In May 2015, the FASB issued ASU No. 2015-09, Financial Services-Insurance (Topic 944): Disclosures aboutShort-Duration Contracts.  The updated accounting guidance requires enhanced disclosures to provide additional information about insurance liabilities for short-duration contracts.  The guidance is effective for annual periods beginning after December 15, 2015, and interim periods within the annual periods beginning after December 15, 2016.  The Company is currently evaluating the effect the updated guidance will have on the Company's financial statement disclosures, but expects to provide additional incurred and paid claims development information by accident year, quantitative information about claim frequency and the history of claims duration for significant lines of business within the Company’s annual financial statements.In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.  The updated accounting guidance requires changes to the reporting model for financial instruments.  The guidance is effective for interim and annual periods beginning after December 15, 2017.  The Company is currently evaluating the effect the guidance will have on the Company's financial statements, and expects the primary change for the Company to be the requirement for equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income.  In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842): Accounting for Leases.  The updated accounting guidance requires lessees to recognize on the balance sheet the assets and liabilities for the rights and obligations created by all leases, including those historically accounted for as operating leases.  The guidance is effective for interim and annual periods beginning after December 15, 2018.  The Company is currently evaluating the effect the updated guidance will have on the Company's financial statements. It is expected that assets and liabilities will increase based on the present value of remaining lease payments for leases in place at the adoption date, however, this is not expected to be material to the Company's results of operations or financial position.In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting.  The updated accounting guidance simplifies the accounting for share-based payment award transactions including income tax consequences, classification of awards as either equity or liabilities and classification on the statement of cash flows.  The guidance is effective for annual and interim periods beginning after December 15, 2016.  The Company is currently evaluating the effect the updated guidance will have on the Company’s financial statements, but anticipates the primary change to be the recognition of excess tax benefits or deficiencies on vesting or settlement of awards as an income tax benefit or expense, respectively, within net income and the related cash flows classified within operating activities.In 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 Company is currently evaluating the effect the guidance will have on the Company's financial statements, but expects the primary changes to be the use of the expected credit loss model for its mortgage loan portfolio and reinsurance receivables and the presentation of credit losses within the available-for-sale fixed maturities portfolio through an allowance method rather than as a direct write-down.  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. |

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| - Definition
The entire disclosure for the general note to the financial statements for the reporting entity which may include, descriptions of the basis of presentation, business description, significant accounting policies, consolidations, reclassifications, new pronouncements not yet adopted and changes in accounting principles.
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