| **General**                                                                                                  | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|--------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Organization, Consolidation and Presentation of Financial Statements [Abstract]**                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Organization, Consolidation, Basis of Presentation, Business Description of 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, 2015.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, 2014, 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 as of the date of the Condensed Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting periods.  Actual results may differ from those estimates.The interim financial data as of September 30, 2015 and for the three and nine months endedSeptember 30, 2015 and 2014 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.Sale of Continental Assurance Company (CAC)On August 1, 2014, the Company completed the sale of the common stock of CAC, the Company's former life insurance subsidiary.  The Company elected to include CAC cash flow activity in the comparative Condensed Consolidated Statement of Cash Flows.  Further information on discontinued operations is provided in Note K to the Condensed Consolidated Financial Statements.In connection with the sale of CAC, the Company entered into a 100% coinsurance agreement on a separate small block of annuity business outside of CAC.  As a result of the coinsurance agreement, the $34 million difference between market value and book value of the funds withheld assets at the coinsurance contract's inception was recognized as a loss in Other operating expenses in the third quarter of 2014.Recently Issued Accounting Standards Update (ASU) - Disclosures about Short-Duration ContractsIn May of 2015, the Financial Accounting Standards Board issued ASU No. 2015-09, Financial Services-Insurance (Topic 944): Disclosures about Short-Duration Contracts.  The updated accounting guidance requires enhanced disclosures to provide additional information about insurance liabilities for short-duration contracts.  The updated guidance is effective for annual financial statements issued for fiscal years 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. |

| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
No definition available.
+ Details**Name:**us-gaap_OrganizationConsolidationBasisOfPresentationBusinessDescriptionAndAccountingPoliciesTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |