| **Related Party Transactions**            | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
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| **Related Party Transactions [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
| Related Party Transactions                | Related Party TransactionsThe Company reimburses Loews for, or pays directly, fees and expenses of investment facilities and services provided to the Company.  Additionally, the Company provides investment-related processing services to Loews and charges Loews for these services.  The net amounts incurred by the Company for these fees, expenses and services were $44 million, $43 million and $43 million for the years ended December 31, 2019, 2018 and 2017.  Net amounts due to Loews related to these services, included in Other liabilities and payable in the first quarter of the subsequent year, were $21 million and $23 million as of December 31, 2019 and 2018.  In addition, the Company reimbursed Loews for general corporate services and related travel expenses of $1 million and less than $1 million for the years ended December 31, 2019 and 2018.  The CNA Tax Group is included in the consolidated federal income tax return of Loews and its eligible subsidiaries.  The related receivable from Loews, included in Other assets, was $21 million and $8 million as of December 31, 2019 and 2018.  For a detailed description of the income tax agreement with Loews see Note D 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 each of the years ended December 31, 2019, 2018 and 2017 were $2 million. |

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| - References
No definition available.
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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
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -URI http://asc.fasb.org/topic&trid=2122745
+ Details**Name:**us-gaap_RelatedPartyTransactionsDisclosureTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |