| **Business Segments (Income Statement Information) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | $ 1,825        | $ 1,781        | $ 5,389 | $ 5,098 |
| Net investment income                                                                                         | 597            | 601            | 1,808 | 1,719 |
| Other revenues                                                                                                | 78             | 76             | 285 | 230 |
| Total operating revenues                                                                                      | 2,500          | 2,458          | 7,482 | 7,047 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 1,408          | 1,430          | 4,348 | 4,150 |
| Policyholders' dividends                                                                                      | 6              | 5              | 16 | 14 |
| Amortization of deferred acquisition costs                                                                    | 341            | 333            | 1,004 | 937 |
| Other insurance related expenses                                                                              | 253            | 268            | 768 | 766 |
| Other expenses                                                                                                | 121            | 116            | 350 | 338 |
| Total claims, benefits and expenses                                                                           | 2,129          | 2,152          | 6,486 | 6,205 |
| Operating income (loss) before income tax                                                                     | 371            | 306            | 996 | 842 |
| Income tax (expense) benefit on operating income (loss)                                                       | (102)          | (90)           | (292) | (248) |
| Net operating income (loss)                                                                                   | 269            | 216            | 704 | 594 |
| Net realized investment gains (losses), pre-tax                                                               | 4              | 8              | 18 | 66 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (1)            | (3)            | (6) | (23) |
| Net realized investment gains (losses)                                                                        | 3              | 5              | 12 | 43 |
| Net income (loss)                                                                                             | 272            | 221            | 716 | 637 |
| CNA Specialty [Member]                                                                                        |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 768            | 738            | 2,237 | 2,163 |
| Net investment income                                                                                         | 159            | 159            | 480 | 446 |
| Other revenues                                                                                                | 67             | 58             | 190 | 171 |
| Total operating revenues                                                                                      | 994            | 955            | 2,907 | 2,780 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 427            | 460            | 1,344 | 1,376 |
| Policyholders' dividends                                                                                      | 3              | 1              | 5 |    |
| Amortization of deferred acquisition costs                                                                    | 161            | 156            | 467 | 458 |
| Other insurance related expenses                                                                              | 64             | 73             | 200 | 222 |
| Other expenses                                                                                                | 58             | 53             | 172 | 153 |
| Total claims, benefits and expenses                                                                           | 713            | 743            | 2,188 | 2,209 |
| Operating income (loss) before income tax                                                                     | 281            | 212            | 719 | 571 |
| Income tax (expense) benefit on operating income (loss)                                                       | (94)           | (76)           | (244) | (197) |
| Net operating income (loss)                                                                                   | 187            | 136            | 475 | 374 |
| Net realized investment gains (losses), pre-tax                                                               | 2              | 2              | (1) | 18 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (1)            | (2)            |    | (6) |
| Net realized investment gains (losses)                                                                        | 1              |      | (1) | 12 |
| Net income (loss)                                                                                             | 188            | 136            | 474 | 386 |
| CNA Commercial [Member]                                                                                       |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 832            | 840            | 2,509 | 2,452 |
| Net investment income                                                                                         | 219            | 230            | 680 | 646 |
| Other revenues                                                                                                | 10             | 11             | 85 | 31 |
| Total operating revenues                                                                                      | 1,061          | 1,081          | 3,274 | 3,129 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 565            | 591            | 1,809 | 1,749 |
| Policyholders' dividends                                                                                      | 1              | 3              | 5 | 9 |
| Amortization of deferred acquisition costs                                                                    | 148            | 151            | 450 | 437 |
| Other insurance related expenses                                                                              | 141            | 146            | 426 | 425 |
| Other expenses                                                                                                | 7              | 7              | 18 | 24 |
| Total claims, benefits and expenses                                                                           | 862            | 898            | 2,708 | 2,644 |
| Operating income (loss) before income tax                                                                     | 199            | 183            | 566 | 485 |
| Income tax (expense) benefit on operating income (loss)                                                       | (68)           | (58)           | (198) | (164) |
| Net operating income (loss)                                                                                   | 131            | 125            | 368 | 321 |
| Net realized investment gains (losses), pre-tax                                                               | 1              | 10             | (7) | 34 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |      | (3)            | 3 | (12) |
| Net realized investment gains (losses)                                                                        | 1              | 7              | (4) | 22 |
| Net income (loss)                                                                                             | 132            | 132            | 364 | 343 |
| Hardy [Member]                                                                                                |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 86             | 64             | 226 | 64 |
| Net investment income                                                                                         | 1              | 2              | 3 | 2 |
| Other revenues                                                                                                | 1              |      | 1 |    |
| Total operating revenues                                                                                      | 88             | 66             | 230 | 66 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 35             | 21             | 107 | 21 |
| Policyholders' dividends                                                                                      |      |      |    |    |
| Amortization of deferred acquisition costs                                                                    | 25             | 20             | 66 | 20 |
| Other insurance related expenses                                                                              | 14             | 13             | 43 | 13 |
| Other expenses                                                                                                | 2              | 7              | 15 | 7 |
| Total claims, benefits and expenses                                                                           | 76             | 61             | 231 | 61 |
| Operating income (loss) before income tax                                                                     | 12             | 5              | (1) | 5 |
| Income tax (expense) benefit on operating income (loss)                                                       |      | (2)            | 3 | (2) |
| Net operating income (loss)                                                                                   | 12             | 3              | 2 | 3 |
| Net realized investment gains (losses), pre-tax                                                               |      | (1)            | 2 | (1) |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | 1              |      |    |    |
| Net realized investment gains (losses)                                                                        | 1              | (1)            | 2 | (1) |
| Net income (loss)                                                                                             | 13             | 2              | 4 | 2 |
| Life and Group Non-Core [Member]                                                                              |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 140            | 141            | 419 | 421 |
| Net investment income                                                                                         | 209            | 201            | 620 | 600 |
| Other revenues                                                                                                | (3)            | 1              | 2 | 15 |
| Total operating revenues                                                                                      | 346            | 343            | 1,041 | 1,036 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 377            | 350            | 1,082 | 1,009 |
| Policyholders' dividends                                                                                      | 2              | 1              | 6 | 5 |
| Amortization of deferred acquisition costs                                                                    | 7              | 6              | 21 | 22 |
| Other insurance related expenses                                                                              | 34             | 35             | 100 | 106 |
| Other expenses                                                                                                | 5              | 8              | 12 | 18 |
| Total claims, benefits and expenses                                                                           | 425            | 400            | 1,221 | 1,160 |
| Operating income (loss) before income tax                                                                     | (79)           | (57)           | (180) | (124) |
| Income tax (expense) benefit on operating income (loss)                                                       | 44             | 35             | 109 | 86 |
| Net operating income (loss)                                                                                   | (35)           | (22)           | (71) | (38) |
| Net realized investment gains (losses), pre-tax                                                               |      | (3)            | 14 | 14 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |      | 1              | (5) | (5) |
| Net realized investment gains (losses)                                                                        |      | (2)            | 9 | 9 |
| Net income (loss)                                                                                             | (35)           | (24)           | (62) | (29) |
| Corporate and Other Non-Core [Member]                                                                         |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           |      | (2)            |    | (1) |
| Net investment income                                                                                         | 9              | 9              | 25 | 25 |
| Other revenues                                                                                                | 3              | 7              | 8 | 14 |
| Total operating revenues                                                                                      | 12             | 14             | 33 | 38 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 4              | 8              | 6 | (5) |
| Policyholders' dividends                                                                                      |      |      |    |    |
| Amortization of deferred acquisition costs                                                                    |      |      |    |    |
| Other insurance related expenses                                                                              | 1              | 1              | 1 | 1 |
| Other expenses                                                                                                | 49             | 42             | 134 | 137 |
| Total claims, benefits and expenses                                                                           | 54             | 51             | 141 | 133 |
| Operating income (loss) before income tax                                                                     | (42)           | (37)           | (108) | (95) |
| Income tax (expense) benefit on operating income (loss)                                                       | 16             | 11             | 38 | 29 |
| Net operating income (loss)                                                                                   | (26)           | (26)           | (70) | (66) |
| Net realized investment gains (losses), pre-tax                                                               | 1              |      | 10 | 1 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (1)            | 1              | (4) |    |
| Net realized investment gains (losses)                                                                        |      | 1              | 6 | 1 |
| Net income (loss)                                                                                             | (26)           | (25)           | (64) | (65) |
| Eliminations [Member]                                                                                         |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | (1)            |      | (2) | (1) |
| Net investment income                                                                                         |      |      |    |    |
| Other revenues                                                                                                |      | (1)            | (1) | (1) |
| Total operating revenues                                                                                      | (1)            | (1)            | (3) | (2) |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              |      |      |    |    |
| Policyholders' dividends                                                                                      |      |      |    |    |
| Amortization of deferred acquisition costs                                                                    |      |      |    |    |
| Other insurance related expenses                                                                              | (1)            |      | (2) | (1) |
| Other expenses                                                                                                |      | (1)            | (1) | (1) |
| Total claims, benefits and expenses                                                                           | (1)            | (1)            | (3) | (2) |
| Operating income (loss) before income tax                                                                     |      |      |    |    |
| Income tax (expense) benefit on operating income (loss)                                                       |      |      |    |    |
| Net operating income (loss)                                                                                   |      |      |    |    |
| Net realized investment gains (losses), pre-tax                                                               |      |      |    |    |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |      |      |    |    |
| Net realized investment gains (losses)                                                                        |      |      |    |    |
| Net income (loss)                                                                                             |      |      |    |    |

| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Net Realized Investment Gains Losses
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Income Tax Expense (Benefit) on Operating Income (Loss)
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| - Definition
The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It also includes realized gains and losses on trading securities.
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| - Definition
Net Operating Income (Loss) from Continuing Operations Attributable to Parent
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Net realized investment gains (losses)
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Operating Income (Loss) from Continuing Operations before Income Tax
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Operating Revenues
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| - Definition
Other Insurance Related Expenses
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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
The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.
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| - Definition
Amount of deferred policy acquisition costs charged to expense in the period, generally in proportion to related revenue earned, estimated gross profits, or over the customer relationship or some other period.
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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
This element represents a sum total of expenses not separately reflected on the income statement for the period.
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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.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Provision for benefits, claims and claims settlement expenses incurred during the period net of the effects of contracts assumed and ceded.
+ References
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| - Definition
Dividends paid or that became payable to policyholders in the period based upon the insurer's obligations under the policies and contracts in force.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Premiums earned on the income statement for all insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and adding premiums assumed from other insurers.
+ References
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| X                                                                                                                                                    |
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