| **Legal Proceedings, Contingencies and Guarantees**                             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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| Legal Proceedings, Contingencies and Guarantees                                 | Legal Proceedings, Contingencies and Guarantees CNA 401(k) Plus Plan LitigationIn September 2016, a class action lawsuit was filed against CCC, Continental Assurance Company (CAC) (a former subsidiary of CCC), CNAF, the Investment Committee of the CNA 401(k) Plus Plan (Plan), The Northern Trust Company and John Does 1-10 (collectively Defendants) related to the Plan.  The complaint alleges that Defendants breached fiduciary duties to the Plan and caused prohibited transactions in violation of the Employee Retirement Income Security Act of 1974 when the Plan's Fixed Income Fund’s annuity contract with CAC was canceled.  The plaintiff alleges he and a proposed class of Plan participants who had invested in the Fixed Income Fund suffered lower returns in their Plan investments as a consequence of these alleged violations and seeks relief on behalf of the putative class.  The Plan trustees have provided notice to their fiduciary coverage insurance carriers.The plaintiff, Defendants and the Plan's fiduciary insurance carriers have agreed on terms to settle this matter.  Upon execution of final settlement agreements, plaintiff and Defendants will propose a class settlement for court approval.  Based on the executed term sheet, management has recorded its best estimate of the Company's probable loss and does not believe that the ultimate resolution of this matter will have a material impact on the Company’s results of operations or financial position. Small Business Premium Rate AdjustmentIn 2016 and 2017, the Company identified rating errors related to its multi-peril package product and workers' compensation policies within its Small Business unit and determined that it would voluntarily issue premium refunds along with interest on affected policies.  After the rating errors were identified, written and earned premium have been reported net of any impact from the premium rate adjustments.  In the first quarter of 2017, the Company recorded a charge which reduced Earned premium by $38 million and increased Interest expense by $5 million for interest due to policyholders on the premium rate adjustments. The policyholder refunds for the multi-peril package product were issued in the third quarter of 2017.  The policyholder refunds for workers’ compensation policies are expected to be refunded in the second half of 2018 and, as such, an additional $1 million of Interest expense was recorded in the first quarter of 2018.  The estimated refund liability, including interest, for the workers' compensation policies as of March 31, 2018 was $60 million. Other LitigationThe Company is a party to other routine litigation incidental to its business, which, based on the facts and circumstances currently known, is not material to the Company's results of operations or financial position.Guarantees As of March 31, 2018 and December 31, 2017, the Company had recorded liabilities of approximately $5 million related to guarantee and indemnification agreements and management believes that it is not likely that any future indemnity claims will be significantly greater than the amounts recorded.In the course of selling business entities and assets to third parties, the Company agreed to guarantee the performance of certain obligations of previously owned subsidiaries and to indemnify purchasers for losses arising out of breaches of representations and warranties with respect to the business entities or assets sold, including, in certain cases, losses arising from undisclosed liabilities or certain named litigation.  Such guarantee and indemnification agreements in effect for sales of business entities, assets and third-party loans may include provisions that survive indefinitely.  As of March 31, 2018, the aggregate amount related to quantifiable guarantees was $375 million and the aggregate amount related to quantifiable indemnification agreements was $252 million.  In certain cases, should the Company be required to make payments under any such guarantee, it would have the right to seek reimbursement from an affiliate of a previously owned subsidiary.In addition, the Company has agreed to provide indemnification to third-party purchasers for certain losses associated with sold business entities or assets that are not limited by a contractual monetary amount.  As of March 31, 2018 the Company had outstanding unlimited indemnifications in connection with the sales of certain of its business entities or assets that included tax liabilities arising prior to a purchaser's ownership of an entity or asset, defects in title at the time of sale, employee claims arising prior to closing and in some cases losses arising from certain litigation and undisclosed liabilities.  Certain provisions of the indemnification agreements survive indefinitely, while others survive until the applicable statutes of limitation expire, or until the agreed-upon contract terms expire.  The Company also provided guarantees, if the primary obligor fails to perform, to holders of structured settlement annuities provided by a previously owned subsidiary.  As of March 31, 2018, the potential amount of future payments the Company could be required to pay under these guarantees was approximately $1.8 billion, which will be paid over the lifetime of the annuitants.  The Company does not believe any payment is likely under these guarantees, as the Company is the beneficiary of a trust that must be maintained at a level that approximates the discounted reserves for these annuities. |

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