| **General (Narrative) (Details) (USD $)In Millions, except Per Share data, unless otherwise specified** | 3 Months Ended | 6 Months Ended |                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------|----------------|----------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Basis of presentation [Member]                                                                          |           |           |                                                                                                                                                                                                                    |   |   |
| Percentage owned of outstanding common stock by majority shareholder                                    |           |           | 90.00%                                                                                                                                                                                                                  |   |   |
| Noncontrolling interest [Member]                                                                        |           |           |                                                                                                                                                                                                                    |   |   |
| Sale of CNA ownership percentage of FICOH                                                               |           |           |                                                                                                                                                                                                                    |   | 50.00% |
| Accounting standards update 2010-26 [Member]                                                            |           |           |                                                                                                                                                                                                                    |   |   |
| 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                                                              | $ 1            | $ 2            | $ 3                                                                                                                                                                                                                     | $ 5 |   |
| Decrease in deferred acquisition costs                                                                  |           |           |                                                                                                                                                                                                                    |   | 106 |
| Increase in deferred income taxes                                                                       |           |           |                                                                                                                                                                                                                    |   | 37 |
| Decrease in amortization of deferred acquisition costs                                                  |           | 64             |                                                                                                                                                                                                                    | 112 |   |
| Increase in other operating expenses                                                                    |           | 67             |                                                                                                                                                                                                                    | 119 |   |
| Changes in income tax expense benefit                                                                   |           |      |                                                                                                                                                                                                                    | 1 |   |
| Decrease in net income attributable to noncontrolling interest                                          |           | 1              |                                                                                                                                                                                                                    | 1 |   |
| Decrease in basic and diluted earnings per share                                                        | $ 0.01         | $ 0.01         | $ 0.01                                                                                                                                                                                                                  | $ 0.02 |   |
| 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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Decrease in basic and diluted earnings per share due to updated accounting guidance concerning deferred acquisition costs
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the effect of a change in accounting principle on Net Income attributable to noncontrolling interest.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage owned of outstanding common stock by majority shareholder
+ References
No definition available.
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| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sale of CNA ownership percentage of FICOH
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the effect of a change in accounting principle on net income.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of why retrospective application is impracticable and the alternative method used to report the change in accounting principle.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Retrospective Application -URI http://asc.fasb.org/extlink&oid=6523989Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6801783&loc=d3e22580-107794Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Indirect Effects of a Change in Accounting Principle -URI http://asc.fasb.org/extlink&oid=6515603Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 154 -Paragraph 2, 17, 18 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=6801783&loc=d3e22499-107794Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Change in Accounting Principle -URI http://asc.fasb.org/extlink&oid=6507316Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accounting Change -URI http://asc.fasb.org/extlink&oid=6503790Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Direct Effects of a Change in Accounting Principle -URI http://asc.fasb.org/extlink&oid=6510796
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