| **Investments (Summary of fixed maturity and equity securities) (Details) (USD $)In Millions, unless otherwise specified** | 6 Months Ended | 12 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| Cost or Amortized Cost                                                                                                     | $ 37,387       | $ 39,490        |
| Gross Unrealized Gains                                                                                                     | 3,401          | 2,460           |
| Gross Unrealized Losses                                                                                                    | 126            | 532             |
| Estimated fair value                                                                                                       | 40,662         | 41,418          |
| Corporate and other bonds [Member]                                                                                         |           |            |
| Cost or Amortized Cost                                                                                                     | 17,276         | 19,352          |
| Gross Unrealized Gains                                                                                                     | 1,852          | 1,645           |
| Gross Unrealized Losses                                                                                                    | 30             | 135             |
| Estimated fair value                                                                                                       | 19,098         | 20,862          |
| Unrealized OTTI Losses (Gains)                                                                                             |      |       |
| States, municipalities and political subdivisions [Member]                                                                 |           |            |
| Cost or Amortized Cost                                                                                                     | 11,215         | 11,281          |
| Gross Unrealized Gains                                                                                                     | 1,178          | 548             |
| Gross Unrealized Losses                                                                                                    | 70             | 272             |
| Estimated fair value                                                                                                       | 12,323         | 11,557          |
| Unrealized OTTI Losses (Gains)                                                                                             |      |       |
| Residential mortgage-backed [Member]                                                                                       |           |            |
| Cost or Amortized Cost                                                                                                     | 4,882          | 4,940           |
| Gross Unrealized Gains                                                                                                     | 202            | 123             |
| Gross Unrealized Losses                                                                                                    | 14             | 92              |
| Estimated fair value                                                                                                       | 5,070          | 4,971           |
| Unrealized OTTI Losses (Gains)                                                                                             | (51)           | (37)            |
| Commercial mortgage-backed [Member]                                                                                        |           |            |
| Cost or Amortized Cost                                                                                                     | 1,999          | 1,995           |
| Gross Unrealized Gains                                                                                                     | 107            | 90              |
| Gross Unrealized Losses                                                                                                    | 7              | 22              |
| Estimated fair value                                                                                                       | 2,099          | 2,063           |
| Unrealized OTTI Losses (Gains)                                                                                             | (3)            | (3)             |
| Other asset-backed [Member]                                                                                                |           |            |
| Cost or Amortized Cost                                                                                                     | 1,172          | 945             |
| Gross Unrealized Gains                                                                                                     | 16             | 13              |
| Gross Unrealized Losses                                                                                                    | 3              | 3               |
| Estimated fair value                                                                                                       | 1,185          | 955             |
| Unrealized OTTI Losses (Gains)                                                                                             |      |       |
| Total asset-backed [Member]                                                                                                |           |            |
| Cost or Amortized Cost                                                                                                     | 8,053          | 7,880           |
| Gross Unrealized Gains                                                                                                     | 325            | 226             |
| Gross Unrealized Losses                                                                                                    | 24             | 117             |
| Estimated fair value                                                                                                       | 8,354          | 7,989           |
| Unrealized OTTI Losses (Gains)                                                                                             | (54)           | (40)            |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                 |           |            |
| Cost or Amortized Cost                                                                                                     | 61             | 139             |
| Gross Unrealized Gains                                                                                                     | 6              | 6               |
| Gross Unrealized Losses                                                                                                    | 1              | 1               |
| Estimated fair value                                                                                                       | 66             | 144             |
| Unrealized OTTI Losses (Gains)                                                                                             |      |       |
| Foreign Government [Member]                                                                                                |           |            |
| Cost or Amortized Cost                                                                                                     | 533            | 531             |
| Gross Unrealized Gains                                                                                                     | 18             | 15              |
| Gross Unrealized Losses                                                                                                    | 1              | 3               |
| Estimated fair value                                                                                                       | 550            | 543             |
| Unrealized OTTI Losses (Gains)                                                                                             |      |       |
| Redeemable preferred stock [Member]                                                                                        |           |            |
| Cost or Amortized Cost                                                                                                     | 39             | 92              |
| Gross Unrealized Gains                                                                                                     | 3              | 10              |
| Gross Unrealized Losses                                                                                                    |      |       |
| Estimated fair value                                                                                                       | 42             | 102             |
| Unrealized OTTI Losses (Gains)                                                                                             |      |       |
| Total fixed maturity securities available-for-sale [Member]                                                                |           |            |
| Cost or Amortized Cost                                                                                                     | 37,177         | 39,275          |
| Gross Unrealized Gains                                                                                                     | 3,382          | 2,450           |
| Gross Unrealized Losses                                                                                                    | 126            | 528             |
| Estimated fair value                                                                                                       | 40,433         | 41,197          |
| Unrealized OTTI Losses (Gains)                                                                                             | (54)           | (40)            |
| Total fixed maturity securities trading [Member]                                                                           |           |            |
| Cost or Amortized Cost, Trading Securities                                                                                 | 35             | 36              |
| Gross Unrealized Gains, Trading Securities                                                                                 |      |       |
| Gross Unrealized Losses, Trading Securities                                                                                |      |       |
| Estimated Fair Value, Trading Securities                                                                                   | 35             | 36              |
| Common stock [Member]                                                                                                      |           |            |
| Cost or Amortized Cost                                                                                                     | 46             | 36              |
| Gross Unrealized Gains                                                                                                     | 14             | 9               |
| Gross Unrealized Losses                                                                                                    |      |       |
| Estimated fair value                                                                                                       | 60             | 45              |
| Preferred stock [Member]                                                                                                   |           |            |
| Cost or Amortized Cost                                                                                                     | 129            | 143             |
| Gross Unrealized Gains                                                                                                     | 5              | 1               |
| Gross Unrealized Losses                                                                                                    |      | 4               |
| Estimated fair value                                                                                                       | 134            | 140             |
| Total equity securities available-for-sale [Member]                                                                        |           |            |
| Cost or Amortized Cost                                                                                                     | 175            | 179             |
| Gross Unrealized Gains                                                                                                     | 19             | 10              |
| Gross Unrealized Losses                                                                                                    |      | 4               |
| Estimated fair value                                                                                                       | $ 194          | $ 185           |

| X                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The accumulated unrealized gain or loss position on Other Than Temporary Impariment (OTTI) Securities
+ References
No definition available.
+ Details**Name:**cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossesBeforeTaxDebtSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Trading Securities, Unrealized Holding Gains
+ References
No definition available.
+ Details**Name:**cna_TradingSecuritiesUnrealizedHoldingGains**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Trading Securities, Unrealized Holding Losses
+ References
No definition available.
+ Details**Name:**cna_TradingSecuritiesUnrealizedHoldingLosses**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789
+ Details**Name:**us-gaap_TradingSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
No definition available.
+ Details**Name:**us-gaap_TradingSecuritiesCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |