| **General**                        | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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| **Accounting Policies [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                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| General                            | 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 March 31, 2017.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 ended December 31, 2016, 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 March 31, 2017 and for the three months ended March 31, 2017 and 2016 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 March 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvementsto 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.  As of January 1, 2017, the Company adopted the updated accounting guidanceand began recognizing excess tax benefits or deficiencies on vesting or settlement of awards as an income tax benefit or expense within net income, instead of additional paid-in capital as required under previous guidance.  The related cash flows are now classified within operating activities.  As a result of this change, excess tax benefits are no longer included in assumed proceeds under the treasury stock method of calculating earnings per share.  The impact of the accounting change resulted in a decrease of $3 million to income tax expense for the three months ended March 31, 2017.Accounting Standards Pending AdoptionIn May 2014, the FASB issued ASU No. 2014-09, Revenue Recognition (Topic 606): Revenue from Contracts with Customers.  The standard excludes from its scope the accounting for insurance contracts, financial instruments, and certain other agreements that are governed under other GAAP guidance. The updated guidance requires an entity to recognize revenue as performance obligations are met, in an amount that reflects the consideration the entity is entitled to receive for the transfer of the promised goods or services. The standard is effective for interim and annual reporting periods beginning after December 15, 2017 and may be applied retrospectively or through a cumulative effect adjustment to retained earnings at the date of adoption.  As insurance contracts are out of scope, the Company anticipates the primary change will be to revenue recognition for certain of our warranty products and services.  The Company has not made a decision on the method of adoption and is currently evaluating the effect the updated guidance will have on the Company’s financial statements, but we do not currently believe ASU 2014-09 will have a material effect on the Company's results of operations or financial position.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 June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurementof 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.In March 2017, the FASB issued ASU No. 2017-07, Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The updated accounting guidance requires changes to the presentation of the components of net periodic benefit cost on the income statement by requiring service cost to be presented with other employee compensation costs and other components of net periodic pension cost to be presented outside of any subtotal of operating income.  The ASU also stipulates that only the service cost component of net benefit cost is eligible for capitalization. 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. |

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| - References
No definition available.
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| - Definition
The entire disclosure for all significant accounting policies of the reporting entity.
+ References
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