| **Related Party Transactions**            | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
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| **Related Party Transactions [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Related Party Transactions                | Note S. Related Party TransactionsThe Company reimburses Loews, or pays directly, for fees and expenses of investment facilities and services provided to the Company.  The amounts incurred by the Company for these fees and expenses were $39 million, $37 million and $36 million for the years ended December 31, 2014, 2013 and 2012 and amounts due to Loews, included in Other liabilities and payable in the first quarter of the subsequent year, were $21 million and $19 million as of December 31, 2014 and 2013.  In addition, the Company reimbursed Loews for general corporate services and related travel expenses of $4 million for the years ended December 31, 2013 and 2012.  The CNA Tax Group is included in the consolidated federal income tax return of Loews and its eligible subsidiaries, and the related payable to Loews, included in Other liabilities, was $132 million and $178 million as of December 31, 2014 and 2013.  For a detailed description of the income tax agreement with Loews see Note E to the Consolidated Financial Statements.  In addition, the Company writes, at standard rates, a limited amount of insurance for Loews and its subsidiaries.  The earned premiums for the years ended December 31, 2014, 2013 and 2012 were $2 million in each year.CNA previously sponsored a stock ownership plan whereby the Company financed the purchase of Company common stock by certain former officers, including executive officers.  Interest charged on the principal amount of these outstanding stock purchase loans is generally equivalent to the short term applicable federal rate in effect on the disbursement date of the loan, compounded semi-annually.  Loans made pursuant to the plan were full recourse and secured by the stock purchased.  The loans were originally issued with a ten-year maturity date, and the remaining loan has been extended with current terms maturing through May 2016. |

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| - Definition
The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.
+ References
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