| **Benefit Plans (Reconciliation of Level 3 Plan Assets) (Details) (Fair Value, Inputs, Level 3 [Member], USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Corporate and other bonds [Member]                                                                                                                       |            |   |
| Fair value of plan assets at January 1                                                                                                                   | $ 10            |    |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            |       |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  |       | 10 |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 10              | 10 |
| Residential Mortgage Backed Securities [Member]                                                                                                          |            |   |
| Fair value of plan assets at January 1                                                                                                                   |            | 52 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            |            |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |            | 6 |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  |            | (58) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |            |    |
| Fair value of plan assets at December 31                                                                                                                 |            |    |
| Commercial Mortgage Backed Securities [Member]                                                                                                           |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 9               |    |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            |       |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (9)             | 9 |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 |       | 9 |
| Other asset-backed [Member]                                                                                                                              |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 1               | 5 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            |       |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (1)             | (4) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 |       | 1 |
| Total asset-backed [Member]                                                                                                                              |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 10              | 57 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            |       |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       | 6 |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (10)            | (53) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 |       | 10 |
| Fixed Maturities [Member]                                                                                                                                |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 20              | 57 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            |       |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       | 6 |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (10)            | (43) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 10              | 20 |
| Equity securities [Member]                                                                                                                               |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 6               | 5 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            | (1)             | 1 |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  |       |    |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 5               | 6 |
| Hedge Funds [Member]                                                                                                                                     |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 394             | 339 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            | 5               | 64 |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    | 5               |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (74)            | (9) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 330             | 394 |
| Private Equity Funds [Member]                                                                                                                            |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 59              | 57 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            | 9               | 6 |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (3)             | (4) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 65              | 59 |
| Total limited partnerships [Member]                                                                                                                      |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 453             | 396 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            | 14              | 70 |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    | 5               |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (77)            | (13) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 395             | 453 |
| Investment Contracts with Insurance Company [Member]                                                                                                     |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 9               | 9 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            | 1               |    |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    |       |    |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  |       |    |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | 10              | 9 |
| Total assets [Member]                                                                                                                                    |            |   |
| Fair value of plan assets at January 1                                                                                                                   | 488             | 467 |
| Defined Benefit Plan, Actual Return on Plan Assets Still Held                                                                                            | 14              | 71 |
| Defined Benefit Plan, Actual Return on Plan Assets Sold During Period                                                                                    | 5               | 6 |
| Defined Benefit Plan, Purchases, Sales, and Settlements                                                                                                  | (87)            | (56) |
| Defined Benefit Plan, Transfers Between Measurement Levels                                                                                               |       |    |
| Fair value of plan assets at December 31                                                                                                                 | $ 420           | $ 488 |

| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of plan assets with unobservable inputs
+ References
No definition available.
+ Details**Name:**cna_PlanAssetValueWithUnobservableInputs**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Actual return on plan assets, separately identifying the amount related to assets sold during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(i) -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.
+ Details**Name:**us-gaap_DefinedBenefitPlanActualReturnOnPlanAssetsSoldDuringPeriod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(i) -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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchases, sales, and settlements of plan assets that occurred during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(B) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(iv)(02)(B) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(ii) -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.
+ Details**Name:**us-gaap_DefinedBenefitPlanPurchasesSalesAndSettlements**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Transfers in and out of Level 3. For example, transfers due to changes in the observability of significant inputs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(iv)(02)(C) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(C) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(iii) -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.
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