| **Hardy (Tables)**                                                | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Hardy [Abstract]**                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Schedule of fair value of assets acquired and liabilities assumed | (In millions)Acquisition DateJuly 2, 2012Investments: Fixed maturity securities$117Other invested assets68Short term investments187Total investments372Cash34Reinsurance receivables252Insurance receivables222Accrued investment income2Property and equipment4Goodwill35Other assets245Total assets acquired$1,166  Claim and claim adjustment expenses$500Unearned premiums249Long term debt30Other liabilities156Total liabilities assumed$935                                       |
| Schedule of intangible assets acquired                            | (In millions)Economic Useful Life Amount at Acquisition Accumulated AmortizationSyndicate capacityIndefinite $55  Total indefinite-lived intangible assets  55        Value of business acquired1 - 4 years 60 $43Trade name8 years 8 —Distribution channel15 years 13 —Total finite-lived intangible assets  81 43Total intangible assets  $136 $43 |

| X                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_HardyAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Schedule of finite and indefinite-lived intangible assets acquired as part of business combination [Table Text Block]
+ References
No definition available.
+ Details**Name:**cna_ScheduleOfFiniteAndIndefiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTableTextBlock**Namespace Prefix:**cna_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of all of the fair values of the purchase price and assets and liabilities acquired in a business combination.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7488404&loc=d3e6927-128479
+ Details**Name:**us-gaap_ScheduleOfPurchasePriceAllocationTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |