| **Business Segments (Balance Sheet Information) (Details) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 |
|------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Reinsurance receivables                                                                                    | $ 6,137       | $ 6,231       |
| Insurance receivables                                                                                      | 2,072         | 1,983         |
| Deferred acquisition costs                                                                                 | 641           | 598           |
| Goodwill                                                                                                   | 151           | 154           |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        | 24,511        | 24,763        |
| Unearned premiums                                                                                          | 3,759         | 3,610         |
| Future policy benefits                                                                                     | 11,469        | 11,475        |
| Policyholders’ funds                                                                                | 154           | 157           |
| CNA Specialty [Member]                                                                                     |          |          |
| Reinsurance receivables                                                                                    | 662           | 665           |
| Insurance receivables                                                                                      | 707           | 673           |
| Deferred acquisition costs                                                                                 | 313           | 300           |
| Goodwill                                                                                                   | 117           | 117           |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        | 6,807         | 6,748         |
| Unearned premiums                                                                                          | 1,770         | 1,685         |
| Future policy benefits                                                                                     |     |     |
| Policyholders’ funds                                                                                | 9             | 8             |
| CNA Commercial [Member]                                                                                    |          |          |
| Reinsurance receivables                                                                                    | 1,170         | 1,155         |
| Insurance receivables                                                                                      | 1,168         | 1,116         |
| Deferred acquisition costs                                                                                 | 287           | 269           |
| Goodwill                                                                                                   |     |     |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        | 11,105        | 11,326        |
| Unearned premiums                                                                                          | 1,629         | 1,569         |
| Future policy benefits                                                                                     |     |     |
| Policyholders’ funds                                                                                | 14            | 15            |
| Hardy [Member]                                                                                             |          |          |
| Reinsurance receivables                                                                                    | 251           | 294           |
| Insurance receivables                                                                                      | 181           | 181           |
| Deferred acquisition costs                                                                                 | 41            | 29            |
| Goodwill                                                                                                   | 34            | 37            |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        | 470           | 521           |
| Unearned premiums                                                                                          | 218           | 222           |
| Future policy benefits                                                                                     |     |     |
| Policyholders’ funds                                                                                |     |     |
| Life and Group Non-Core [Member]                                                                           |          |          |
| Reinsurance receivables                                                                                    | 1,273         | 1,273         |
| Insurance receivables                                                                                      | 13            | 9             |
| Deferred acquisition costs                                                                                 |     |     |
| Goodwill                                                                                                   |     |     |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        | 3,031         | 3,006         |
| Unearned premiums                                                                                          | 143           | 134           |
| Future policy benefits                                                                                     | 11,469        | 11,475        |
| Policyholders’ funds                                                                                | 131           | 134           |
| Corporate and Other Non-Core [Member]                                                                      |          |          |
| Reinsurance receivables                                                                                    | 2,781         | 2,844         |
| Insurance receivables                                                                                      | 3             | 4             |
| Deferred acquisition costs                                                                                 |     |     |
| Goodwill                                                                                                   |     |     |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        | 3,098         | 3,162         |
| Unearned premiums                                                                                          |     |     |
| Future policy benefits                                                                                     |     |     |
| Policyholders’ funds                                                                                |     |     |
| Eliminations [Member]                                                                                      |          |          |
| Reinsurance receivables                                                                                    |     |     |
| Insurance receivables                                                                                      |     |     |
| Deferred acquisition costs                                                                                 |     |     |
| Goodwill                                                                                                   |     |     |
| **Insurance reserves**                                                                                     |          |          |
| Claim and claim adjustment expenses                                                                        |     |     |
| Unearned premiums                                                                                          | (1)           |     |
| Future policy benefits                                                                                     |     |     |
| Policyholders’ funds                                                                                |     |     |

| X                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gross reinsurance receivable
+ References
No definition available.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of deferred policy acquisition costs capitalized on contracts remaining in force as of the balance sheet date.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date, which is the cumulative amount paid and (if applicable) the fair value of any noncontrolling interest in the acquiree, adjusted for any amortization recognized prior to the adoption of any changes in generally accepted accounting principles (as applicable) and for any impairment charges, in excess of the fair value of net assets acquired in one or more business combination transactions.
+ References
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| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The sum of the known and estimated amounts payable as of the balance sheet date to policyholders pertaining to insured events for long-duration contracts, which can be viewed as either (a) the present value of future benefits to be paid to or on behalf of policyholders and expenses less the present value of future net premiums payable under the insurance contracts or (b) the accumulated amount of net premiums already collected less the accumulated amount of benefits and expenses already paid to or on behalf of policyholders.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The liability as of the balance sheet date to the policyholders for deposits held under the terms of insurance contracts.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gross amount, as of the balance sheet date, due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, before deducting the allowance for doubtful accounts.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of premiums written on insurance contracts that have not been earned as of the balance sheet date.
+ References
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