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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 89.6% of the outstanding common stock of CNAF as of March 31, 2022.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, 2021, 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, 2022 and for the three months ended March 31, 2022 and 2021 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. 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.  For the Company, this includes the Long term care and fully-ceded single premium immediate annuity business.  Entities will be required to review, and update if there is a change, cash flow assumptions (including morbidity and persistency) at least annually, and to 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.  The guidance is effective for interim and annual periods beginning after December 15, 2022, with early adoption permitted, and may be applied using either a modified retrospective transition method or a full retrospective transition method.  Financial statements for prior periods presented shall be adjusted to reflect the effects of applying the new accounting guidance.The Company will adopt the new guidance effective January 1, 2023, using the modified retrospective method applied as of the transition date of January 1, 2021.  The Company will use a published spot rate curve constructed from A+, A and A- rated U.S. dollar denominated corporate bonds matched to the duration of the corresponding insurance liabilities, to calculate discount rates.  The Company will group its long-duration contracts into calendar year cohorts based on the contract issue date and product line.  Long term care contracts will be grouped separately from the Company’s fully-ceded single premium immediate annuity contracts. The most significant impact at the transition date will be the effect of updating the discount rate assumption to reflect an upper-medium grade fixed-income instrument yield, which will be partially offset by the de-recognition of Shadow Adjustments associated with long-duration contracts.  The Company expects the net impact of these changes will be a $2.2 billion - $2.5 billion decrease in Accumulated other comprehensive income as of the transition date of January 1, 2021.  There is a minimal transition impact expected to retained earnings. The requirement to review, and update if there is a change, cash flow assumptions at least annually is expected to change the pattern of earnings being recognized.  Adoption will also significantly expand the Company’s disclosures, and will impact systems, processes, and controls.  While the requirements of the new guidance represent a material change from existing GAAP, the new guidance will not impact capital and surplus under statutory accounting practices, cash flows, or the underlying economics of the business.The Company continues to make progress in connection with these matters and is in process of refining key accounting policy decisions, technology solutions and updates to internal controls associated with adoption of the new guidance.  These in-progress activities include modifications of actuarial valuation systems, data sourcing, analytical procedures and reporting processes. |

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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
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=123372394&loc=d3e18726-107790Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -URI http://asc.fasb.org/topic&trid=2122369
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