| **Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Narrative) (Details) - USD ($)**                                                                           |               | 3 Months Ended | 12 Months Ended | 13 Months Ended |                |               |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|----------------|-----------------|-----------------|----------------|---------------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |          |           |            |            |           |          |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                                                   |          |           | $ 11,000,000    | $ (20,000,000)  | $ (73,000,000) |          |   |   |
| Funds held under reinsurance agreements, liability                                                                                                                               | $ 690,000,000 |           | 4,000,000,000   | 3,300,000,000   |           | $ 690,000,000 |   |   |
| Retroactive reinsurance agreement, maximum                                                                                                                                       | 1,000,000,000 |           |            |            |           | 1,000,000,000 |   |   |
| Reinsurance premiums paid                                                                                                                                                        | $ 697,000,000 |           |            |            |           | $ 64,000,000  |   |   |
| Reinsurance, loss on uncollectible accounts in period, amount                                                                                                                    |          | $ 12,000,000   |            |            |           |          |   |   |
| Securities held as collateral, at fair value                                                                                                                                     |          |           | 634,000,000     |            |           |          |   |   |
| Corporate & Other                                                                                                                                                            |          |           |            |            |           |          |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |          |           |            |            |           |          |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                                                   |          |           | 60,000,000      | 50,000,000      | 38,000,000     |          |   |   |
| Commercial                                                                                                                                                                       |          |           |            |            |           |          |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |          |           |            |            |           |          |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                                                   |          |           | (6,000,000)     | (7,000,000)     | (40,000,000)   |          |   |   |
| Asbestos and Environmental Reserves                                                                                                                                              |          |           |            |            |           |          |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |          |           |            |            |           |          |   |   |
| Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO                                                                                                 |          |           |            |            |           |          |   | $ 1,600,000,000 |
| Aggregate limit under A&EP Loss Portfolio Transfer                                                                                                                           |          |           |            |            |           |          |   | 4,000,000,000 |
| A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer |          |           |            |            |           |          |   | 1,200,000,000 |
| Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer                                                                                                          |          |           |            |            |           |          |   | 2,000,000,000 |
| Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer                                                                                           |          |           |            |            |           |          |   | 215,000,000 |
| Total consideration paid to NICO under AEP Loss Portfolio Transfer                                                                                                               |          |           |            |            |           |          | $ 2,200,000,000 | $ 2,200,000,000 |
| Net A&EP adverse development before consideration of LPT                                                                                                                     |          |           | 143,000,000     | 125,000,000     | 150,000,000    |          |   |   |
| Provision for uncollectible third-party reinsurance, released                                                                                                                    |          |           | 5,000,000       | 25,000,000      | $ 25,000,000   |          |   |   |
| Cumulative amounts ceded under AEP Loss Portfolio Transfer                                                                                                                       |          |           | 3,400,000,000   | 3,300,000,000   |           |          |   |   |
| Deferred reinsurance benefit yet to be recognized                                                                                                                                |          |           | 429,000,000     | $ 398,000,000   |           |          |   |   |
| Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer                                                                                |          |           | 3,100,000,000   |            |           |          |   |   |
| Excess Workers' Compensation LPT                                                                                                                                                 |          |           |            |            |           |          |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |          |           |            |            |           |          |   |   |
| Funds held under reinsurance agreements, liability                                                                                                                               |          |           | $ 690,000,000   |            |           |          |   |   |

| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate limit under A&EP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Claim and allocated claim adjustment expense reserves under existing third party reinsurance contracts.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cumulative amounts ceded under AEP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_CumulativeamountscededunderAEPLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferred reinsurance benefit yet to be recognized
+ References
No definition available.
+ Details**Name:**cna_Deferredreinsurancebenefityettoberecognized**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_FairValueOfCollateralTrustAccountEstablishedByNicoUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net AEP adverse development before consideration of LPT
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_NetreinsurancereceivablestransferredtoNICOunderAEPLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Provision For Uncollectible Third-Party Reinsurance, Released
+ References
No definition available.
+ Details**Name:**cna_ProvisionForUncollectibleThirdPartyReinsuranceReleased**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reinsurance Premiums Paid
+ References
No definition available.
+ Details**Name:**cna_ReinsurancePremiumsPaid**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Retroactive Reinsurance Agreement, Maximum
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total consideration paid to NICO under AEP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_TotalconsiderationpaidtoNICOunderAEPLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of funds held as security for payments to be received under reinsurance contracts.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the write-down or write-off of a receivable from a reinsurer.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5086-111524
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|---------------------------------------------------------------------------------------------------------------------------------------------------------|
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