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| **Organization, Consolidation and Presentation of Financial Statements [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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| General                                                                             | Note A. GeneralBasis of PresentationThe Condensed Consolidated Financial Statements (Unaudited) include the accounts of CNA Financial Corporation (CNAF) and its controlled subsidiaries. Collectively, CNAF and its controlled subsidiaries are referred to as CNA or the Company. CNA’s property and casualty and remaining life and group insurance operations are primarily conducted by Continental Casualty Company (CCC), The Continental Insurance Company, Western Surety Company and Continental Assurance Corporation.  Loews Corporation (Loews) owned approximately 90% of the outstanding common stock of CNAF as of March 31, 2012.The accompanying Condensed Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP).  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 (SEC) for the year ended December 31, 2011, 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 periods.  Actual results may differ from those estimates.The interim financial data as of March 31, 2012 and for the three months ended March 31, 2012 and 2011 is unaudited.  However, in the opinion of management, the interim data includes all adjustments, consisting of normal recurring accruals, 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.  Intercompany amounts have been eliminated.Hardy Underwriting Bermuda Limited (Hardy)On March 21, 2012, CNA announced an agreement to acquire Hardy, a specialized Lloyd’s underwriter, in a cash acquisition for approximately $227 million.  Hardy underwrote approximately $430 million in gross written premium in 2011.  Subject to regulatory approvals and other conditions, the acquisition is expected to be completed during the second quarter of 2012.  As of March 31, 2012, $230 million of the Company's short-term investments were held in escrow in British pounds to fund the acquisition.Noncontrolling InterestsNet income attributable to noncontrolling interests for the three months ended March 31, 2011 represented the noncontrolling interests in CNA Surety Corporation (Surety) and First Insurance Company of Hawaii (FICOH).  On June 10, 2011, CNA completed the acquisition of the noncontrolling interest of Surety and on November 29, 2011, CNA completed the sale of its 50% ownership interest in FICOH.Accounting for Costs Associated with Acquiring or Renewing Insurance ContractsIn October 2010, the Financial Accounting Standards Board issued updated accounting guidance which limits the capitalization of costs incurred to acquire or renew insurance contracts to those that are incremental direct costs of successful contract acquisitions.  The previous guidance allowed the capitalization of acquisition costs that vary with and are primarily related to the acquisition of new and renewal insurance contracts, whether the costs related to successful or unsuccessful efforts.As of January 1, 2012, the Company adopted the updated accounting guidance prospectively as of January 1, 2004, the earliest date practicable.  Due to the lack of available historical data related to certain accident and health contracts issued prior to January 1, 2004, a full retrospective application of the change in accounting guidance was impracticable.  Acquisition costs capitalized prior to January 1, 2004 will continue to be accounted for under the previous accounting guidance and will be amortized over the premium-paying period of the related policies using assumptions consistent with those used for computing future policy benefit reserves for such contracts.For the three month period ended March 31, 2012, the adoption of the new accounting guidance resulted in an approximate $2 million decrease in Net income attributable to CNA and a $0.01 decrease in Basic and diluted earnings per share attributable to CNA common stockholders.The Company has adjusted its previously reported financial information included herein to reflect the change in accounting guidance for deferred acquisition costs.  The impacts of adopting the new accounting standard on the Company's Condensed Consolidated Balance Sheet as of December 31, 2011 were a $106 million decrease in Deferred acquisition costs and a $37 million increase in Deferred income taxes.  The impacts to Accumulated other comprehensive income (AOCI) and Additional paid-in capital (APIC) were the result of the indirect effects of the Company's adoption of this guidance on Shadow Adjustments, as further discussed in Note C, and the Company's acquisition of the noncontrolling interest of Surety as discussed above.The impacts on the Company's Condensed Consolidated Statement of Operations for the three month period ended March 31, 2011 were a $48 million decrease in Amortization of deferred acquisition costs, a $52 million increase in Other operating expenses, and a $1 million decrease in Income tax expense, resulting in a $3 million decrease in Net income attributable to CNA, and a $0.01 decrease in Basic and diluted earnings per share attributable to CNA common stockholders.  There were no changes to net cash flows from operating, investing or financing activities for the comparative period presented as a result of the adoption of the new accounting standard. |

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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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No definition available.
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