| **General (Narrative) (Details) (USD $)In Millions, except Per Share data, unless otherwise specified** |        | 12 Months Ended |        |        | 3 Months Ended                                                                                                                                                                                                          |              |
|---------------------------------------------------------------------------------------------------------|--------|-----------------|--------|--------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------|
||
| Noncontrolling interest, ownership percentage by parent                                                 | 90.00% |            |   |   |                                                                                                                                                                                                                    |         |   |
| Cash acquisition of common stock                                                                        |   |            | $ 227  |   |                                                                                                                                                                                                                    |         |   |
| Gross written premiums written by Hardy                                                                 |   | 430             |   |   |                                                                                                                                                                                                                    |         |   |
| Restricted short-term investments held in escrow                                                        |   |            | 230    |   |                                                                                                                                                                                                                    |         |   |
| Sale of CNA ownership percentage of FICOH                                                               |   |            |   | 50.00% |                                                                                                                                                                                                                    |         |   |
| New Accounting Pronouncement or Change in Accounting Principle, Retrospective Application Impracticable |   |            |   |   | Due to the lack of available historical data related to certain accident and health contracts issued prior to January 1, 2004, a full retrospective application of the change in accounting guidance was impracticable. |         |   |
| Decrease in net income attributable to CNA                                                              |   |            |   |   | 2                                                                                                                                                                                                                       | 3            |   |
| Decrease in basic and diluted earnings per share                                                        |   |            |   |   | $ 0.01                                                                                                                                                                                                                  | $ 0.01       |   |
| Decrease in deferred acquisition costs                                                                  |   |            |   |   |                                                                                                                                                                                                                    |         | 106 |
| Increase in deferred income taxes                                                                       |   |            |   |   |                                                                                                                                                                                                                    |         | 37 |
| Decrease in amortization of deferred acquisition costs                                                  |   |            |   |   |                                                                                                                                                                                                                    | 48           |   |
| Increase in other operating expenses                                                                    |   |            |   |   |                                                                                                                                                                                                                    | 52           |   |
| Decrease in income tax expense                                                                          |   |            |   |   |                                                                                                                                                                                                                    | 1            |   |
| Change in net cash flows from operating, investing and financing activities                             |   |            |   |   |                                                                                                                                                                                                                    |    |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Changes in the sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations. These changes are due to updated accounting guidance concerning deferred acquisition costs.
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Changes in net cash provided by (used in) operating, investing and financing activities. These changes are due to updated accounting guidance concerning deferred acquisition costs.
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| - Definition
Decrease in the 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. This decrease is due to updated accounting guidance concerning deferred acquisition costs.
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Decrease in the net amount of deferred policy acquisition costs capitalized on contracts remaining in force as of the balance sheet date. These changes are due to updated accounting guidance concerning deferred acquisition costs.
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| - Definition
Decrease in the amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. This decrease is due to updated accounting guidance concerning deferred acquisition costs.
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| - Definition
For entities that net deferred tax assets and tax liabilities, represents the unclassified net amount of deferred tax assets and liabilities as of the balance sheet date, which result from applying the applicable enacted tax rate to net temporary differences and carryforwards pertaining to assets or liabilities. A temporary difference is a difference between the tax basis of an asset or liability and its carrying amount in the financial statements prepared in accordance with generally accepted accounting principles that will reverse in ensuing periods. This is the increase in this Balance Sheet account, which is due to updated accounting guidance concerning deferred acquisition costs.
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| - Definition
Increase in the total amount of other operating cost and expense items that are associated with the entity's normal revenue producing operation. This increase is due to updated accounting guidance concerning deferred acquisition costs.
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| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sale of CNA ownership percentage of FICOH
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| - Definition
Amount of cash paid to acquire the entity.
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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the effect of a change in accounting principle on net income.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of why retrospective application is impracticable and the alternative method used to report the change in accounting principle.
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| - Definition
The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.
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| - Definition
Amount of actual premium written by property-casualty insurance underwriter.
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