| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Incurred Claims and Allocated Claim Adjustment Expenses Commercial - Workers Compensation (Details) - Commercial - Workers Compensation [Member] $ in Millions** | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2008 USD ($) | Dec. 31, 2007 USD ($) |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 1,068               |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 5,875               |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2007 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 71,049                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 32                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 627                 | $ 608                 | $ 610                 | $ 604                 | $ 603                 | $ 603                 | $ 604                 | $ 596                 | $ 580                 | $ 568                 |
| Short-duration Insurance Contracts, Accident Year 2008 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 59,883                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 36                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 638                 | 630                   | 622                   | 612                   | 608                   | 606                   | 593                   | 575                   | $ 558                 |                  |
| Short-duration Insurance Contracts, Accident Year 2009 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 51,111                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 46                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 625                 | 617                   | 611                   | 600                   | 596                   | 594                   | 587                   | $ 583                 |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2010 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 48,056                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 45                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 717                 | 697                   | 683                   | 663                   | 641                   | 619                   | $ 576                 |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2011 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 44,571                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 52                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 666                 | 642                   | 648                   | 637                   | 628                   | $ 593                 |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2012 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 41,683                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 86                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 671                 | 661                   | 648                   | 616                   | $ 589                 |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2013 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 38,102                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 121                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 610                 | 584                   | 563                   | $ 528                 |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2014 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 32,996                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 157                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 474                 | 474                   | $ 459                 |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2015 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 31,296                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 206                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 426                 | $ 416                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2016 [Member]                                                                                                                                                                               |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                                   | 27,042                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                                    | $ 287                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                               | $ 421                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

| X                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ References
No definition available.
+ Details**Name:**cna_NetIncurredClaimandClaimAdjustmentExpenseLineItems**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Reported Claims per Million of Net Earned Premium
+ References
No definition available.
+ Details**Name:**cna_NumberofReportedClaimsperMillionofNetEarnedPremium**Namespace Prefix:**cna_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671317-158438Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671321-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsIncurredButNotReportedIbnrClaimsLiabilityNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsIncurredClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialWorkersCompensationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2007Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2008Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2009Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2010Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2011Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2012Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2013Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2014Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2015Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2016Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |