| **Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Statements of Operations and Comprehensive Income) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Revenues**                                                                                                                                                                                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Net realized investment gains (losses)                                                                                                                                                                                                                  |           |            |   |   |   |   |   |   | $ 31 | $ 63 | $ (2) |
| Other revenues                                                                                                                                                                                                                                          |           |            |   |   |   |   |   |   | 361 | 320 | 294 |
| Total revenues                                                                                                                                                                                                                                          | 2,613          | 2,504           | 2,493 | 2,503 | 2,434 | 2,466 | 2,246 | 2,401 | 10,113 | 9,547 | 8,949 |
| **Expenses**                                                                                                                                                                                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Interest                                                                                                                                                                                                                                                |           |            |   |   |   |   |   |   | 166 | 170 | 175 |
| Income tax expense (benefit)                                                                                                                                                                                                                            |           |            |   |   |   |   |   |   | 376 | 244 | 242 |
| Net income (loss) attributable to CNA                                                                                                                                                                                                                   | 221            | 272             | 194 | 250 | (9) | 221 | 166 | 250 | 937 | 628 | 612 |
| Total comprehensive income (loss) attributable to CNA                                                                                                                                                                                                   |           |            |   |   |   |   |   |   | 548 | 979 | 755 |
| Parent Company [Member]                                                                                                                                                                                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| **Revenues**                                                                                                                                                                                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Net investment income                                                                                                                                                                                                                                   |           |            |   |   |   |   |   |   | 1 | 1 | 1 |
| Net realized investment gains (losses)                                                                                                                                                                                                                  |           |            |   |   |   |   |   |   | 4 | 4 | (9) |
| Other revenues                                                                                                                                                                                                                                          |           |            |   |   |   |   |   |   |    | 9 | 40 |
| Total revenues                                                                                                                                                                                                                                          |           |            |   |   |   |   |   |   | 5 | 14 | 32 |
| **Expenses**                                                                                                                                                                                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Administrative and general                                                                                                                                                                                                                              |           |            |   |   |   |   |   |   | (7) | 1 | 3 |
| Interest                                                                                                                                                                                                                                                |           |            |   |   |   |   |   |   | 165 | 164 | 167 |
| Total expenses                                                                                                                                                                                                                                          |           |            |   |   |   |   |   |   | 158 | 165 | 170 |
| Income (loss) from operations before income taxes and equity in net income of subsidiaries                                                                                                                                                              |           |            |   |   |   |   |   |   | (153) | (151) | (138) |
| Income tax expense (benefit)                                                                                                                                                                                                                            |           |            |   |   |   |   |   |   | (22) | (144) | (46) |
| Income (loss) before equity in net income (loss) of subsidiaries                                                                                                                                                                                        |           |            |   |   |   |   |   |   | (131) | (7) | (92) |
| Equity in net income of subsidiaries                                                                                                                                                                                                                    |           |            |   |   |   |   |   |   | 1,068 | 635 | 704 |
| Net income (loss) attributable to CNA                                                                                                                                                                                                                   |           |            |   |   |   |   |   |   | 937 | 628 | 612 |
| Equity in other comprehensive income of subsidiaries                                                                                                                                                                                                    |           |            |   |   |   |   |   |   | (389) | 351 | 143 |
| Total comprehensive income (loss) attributable to CNA                                                                                                                                                                                                   |           |            |   |   |   |   |   |   | $ 548 | $ 979 | $ 755 |

| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Equity in net (income) loss of subsidiaries
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Equity in other comprehensive income of subsidiaries
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| X                                                                                                                                                                                                                                                                                                                            |
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| - Definition
Income Loss From Continuing Operations Before Equity Method Investments
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| - Definition
Income loss from continuing operations before income taxes and income loss from subsidiaries
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.
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| - Definition
The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
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| - Definition
Amount of the cost of borrowed funds accounted for as interest expense.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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| - Definition
Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
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| - Definition
Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.
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| - Definition
Reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement.
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| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
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