| **Investments (Net change in unrealized gains (losses) on investments) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Net Change In Unrealized Pretax Gains (Losses)**                                                                                |            |   |   |
| Net change in unrealized gains (losses) on investments                                                                            | $ 1,437         | $ 1,146 | $ 5,430 |
| Fixed maturity securities [Member]                                                                                                |            |   |   |
| **Net Change In Unrealized Pretax Gains (Losses)**                                                                                |            |   |   |
| Net change in unrealized gains (losses) on investments                                                                            | 1,442           | 1,140 | 5,278 |
| Equity securities [Member]                                                                                                        |            |   |   |
| **Net Change In Unrealized Pretax Gains (Losses)**                                                                                |            |   |   |
| Net change in unrealized gains (losses) on investments                                                                            | (2)             | 7 | 156 |
| Other [Member]                                                                                                                    |            |   |   |
| **Net Change In Unrealized Pretax Gains (Losses)**                                                                                |            |   |   |
| Net change in unrealized gains (losses) on investments                                                                            | $ (3)           | $ (1) | $ (4) |

| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_NetChangeInUnrealizedPretaxGainsLossesAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
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| - Definition
This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 21 -Subparagraph d -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |