| **Subsequent Event**             | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| **Subsequent Events [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Subsequent Event                 | Subsequent EventsCNA Employee Retirement Plan Trust Settlement TransactionIn October of 2024, a subsidiary of CNAF, as a sponsor of the CNA Employee Retirement Plan Trust (the Plan), entered into a commitment agreement, with Metropolitan Life Insurance Company (the Insurer) under which the Plan purchased a nonparticipating single premium group annuity contract that transferred to the Insurer approximately $1,045 million of the Plan’s defined benefit pension obligations.  The group annuity contract covers approximately 7,600 Plan participants and beneficiaries (the Transferred Participants), representing approximately 60% of the Plan’s obligations.  Under the group annuity contract, the Insurer has made an irrevocable commitment, and will be solely responsible, to pay the pension benefits of each Transferred Participant that are due on and after January 1, 2025.  The purchase of the group annuity contract was funded directly by assets of the Plan and required no cash or asset contributions of the Company.  As a result of the transaction, the Company will recognize a one-time, non-cash, pretax pension settlement charge of approximately $370 million ($290 million after-tax) in the fourth quarter of 2024.  This charge is largely driven by the accelerated recognition of the Company’s actuarial pension loss from accumulated other comprehensive income into net income, which does not impact stockholder’s equity.  This charge will not impact the Company’s fourth quarter or full year 2024 core income (loss) or cash flow.   Fourth Quarter 2024 Hurricane Milton EstimatesOn October 9, 2024, Hurricane Milton made landfall in Florida.  Pretax net catastrophe losses related to Hurricane Milton are currently estimated between approximately $25 million to $55 million, and are anticipated to be reflected in the Company's fourth quarter 2024 results. |

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| - Definition
The entire disclosure for significant events or transactions that occurred after the balance sheet date through the date the financial statements were issued or the date the financial statements were available to be issued. Examples include: the sale of a capital stock issue, purchase of a business, settlement of litigation, catastrophic loss, significant foreign exchange rate changes, loans to insiders or affiliates, and transactions not in the ordinary course of business.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 855 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org/855/tableOfContentReference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 855 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483399/855-10-50-2
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