| **Investments (Narrative) (Details) - USD ($) $ in Millions**                                                                                       | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Investments [Abstract]**                                                                                                                          |          |          |          |
| Non-income producing fixed maturity securities                                                                                                      | $ 54          | $ 0           |          |
| Investments that exceed ten percent of stockholders' equity                                                                                         | 0             | 0             |          |
| Fixed maturity securities and equity securities in the energy and metals and mining sectors, at fair value                                          | 2,533         |          |          |
| Fixed maturity securities and equity securities in the energy and metals and mining sectors, amortized cost or cost basis                           | 2,658         |          |          |
| Fixed maturity securities and equity securities in the energy and metals and mining sectors, at fair value, unrealized loss position                | 1,340         |          |          |
| Fixed maturity securities and equity securities in the energy and metals and mining sectors, amortized cost or cost basis, unrealized loss position | 1,554         |          |          |
| Limited partnership investments                                                                                                                     | 2,548         | 2,937         |          |
| Undistributed earnings of limited partnership investments                                                                                           | $ 687         | $ 1,040       |          |
| Percentage of limited partnerships reported on a current basis                                                                                      | 63.00%        |          |          |
| Percentage of limited partnerships reported on a one month lag                                                                                      | 16.00%        |          |          |
| Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments                                   | 70.00%        | 73.00%        |          |
| Limited partnerships invested in private debt and equity                                                                                            | 26.00%        | 23.00%        |          |
| Percentage of equity related limited partnership hedge fund strategies                                                                              | 54.00%        |          |          |
| Percentage of multistrategy approach limited partnership hedge fund strategies                                                                      | 28.00%        |          |          |
| Percentage of distressed investments limited partnership hedge fund strategies                                                                      | 16.00%        |          |          |
| Percentage of fixed income limited partnership hedge fund strategies                                                                                | 2.00%         |          |          |
| Carrying value of ten largest limited partnerships                                                                                                  | $ 1,221       | $ 1,492       |          |
| Carrying value of limited partnerships as percentage of aggregate partnership equity                                                                | 3.00%         | 4.00%         |          |
| Income As Percentage Of Change In Partnership Equity For All Limited Partnerships                                                                   | 3.00%         | 4.00%         | 4.00%         |
| Derivative, Collateral, Right to Reclaim Cash                                                                                                       | $ 0           | $ 0           |          |
| Derivative, Collateral, Obligation to Return Cash                                                                                                   | 0             | 0             |          |
| Commercial mortgage loans past due or in non-accrual status                                                                                         | 0             | 0             |          |
| Valuation allowances recorded on commercial mortgage loans                                                                                          | 0             | 0             |          |
| Future capital call commitments for limited partnership investments                                                                                 | 398           |          |          |
| Mortgage loan commitments                                                                                                                           | 12            |          |          |
| Commitments to purchase or fund privately placed debt securities                                                                                    | 138           |          |          |
| Commitments to sell various privately placed debt securities                                                                                        | 67            |          |          |
| Carrying value of securities deposited under requirements of regulatory authorities                                                                 | 2,800         | 3,000         |          |
| Cash and securities deposited as collateral for letters of credit                                                                                   | 364           | 361           |          |
| Carrying value of assets deposited to secure reinsurance and third party obligations                                                                | $ 263         | $ 302         |          |

| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying Value of Assets Deposited to Secure Reinsurance and Third Party Obligations
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfAssetsDepositedToSecureReinsuranceAndThirdPartyObligations**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying value Of Limited Partnerships As Percentage of Aggregate Partnership Equity
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfLimitedPartnershipsAsPercentageOfAggregatePartnershipEquity**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying Value Of Securities Deposited Under Requirements Of Regulatory Authorities
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfSecuritiesDepositedUnderRequirementsOfRegulatoryAuthorities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying Value Of Ten Largest Limited Partnerships
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfTenLargestLimitedPartnerships**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash And Securities Deposited As Collateral For Letters Of Credit
+ References
No definition available.
+ Details**Name:**cna_CashAndSecuritiesDepositedAsCollateralForLettersOfCredit**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments To Purchase Or Fund Additional Amounts
+ References
No definition available.
+ Details**Name:**cna_CommitmentsToPurchaseOrFundAdditionalAmounts**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments To Sell Various Privately Placed Debt Securities
+ References
No definition available.
+ Details**Name:**cna_CommitmentsToSellVariousPrivatelyPlacedDebtSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fixed maturity securities and equity securities in the energy and metals and mining sectors, amortized cost or cost basis
+ References
No definition available.
+ Details**Name:**cna_Fixedmaturitysecuritiesandequitysecuritiesintheenergyandmetalsandminingsectorsamortizedcostorcostbasis**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fixed maturity securities and equity securities in the energy and metals and mining sectors, amortized cost or cost basis, unrealized loss position
+ References
No definition available.
+ Details**Name:**cna_Fixedmaturitysecuritiesandequitysecuritiesintheenergyandmetalsandminingsectorsamortizedcostorcostbasisunrealizedlossposition**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fixed maturity securities and equity securities in the energy and metals and mining sectors, at fair value
+ References
No definition available.
+ Details**Name:**cna_Fixedmaturitysecuritiesandequitysecuritiesintheenergyandmetalsandminingsectorsatfairvalue**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fixed maturity securities and equity securities in the energy and metals and mining sectors, at fair value, unrealized loss position
+ References
No definition available.
+ Details**Name:**cna_Fixedmaturitysecuritiesandequitysecuritiesintheenergyandmetalsandminingsectorsatfairvalueunrealizedlossposition**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Future capital call commitments to limited partnerships for which we would receive an ownership interest.
+ References
No definition available.
+ Details**Name:**cna_FutureCapitalCallCommitments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships
+ References
No definition available.
+ Details**Name:**cna_IncomeAsPercentageOfChangeInPartnershipEquityForAllLimitedPartnerships**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investments that Exceed Ten Percent of Stockholders' Equity, other than investments in U.S. Treasury and U.S. Government agency securities.
+ References
No definition available.
+ Details**Name:**cna_InvestmentsthatExceedTenPercentofStockholdersEquity**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the carrying amount on the entity's balance sheet of its investment in limited partnerships.
+ References
No definition available.
+ Details**Name:**cna_LimitedPartnershipInvestments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Limited Partnerships Invested In Private Debt and Equity
+ References
No definition available.
+ Details**Name:**cna_LimitedPartnershipsInvestedInPrivateDebtAndEquity**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Mortgage loan commitments, representing signed loan applications received and accepted.
+ References
No definition available.
+ Details**Name:**cna_MortgageLoanCommitments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Non-Income Producing Fixed Maturity Securities
+ References
No definition available.
+ Details**Name:**cna_NonIncomeProducingFixedMaturitySecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Distressed Investments Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfDistressedInvestmentsLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Equity Related Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfEquityRelatedLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Fixed Income Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfFixedIncomeLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments
+ References
No definition available.
+ Details**Name:**cna_PercentageOfLimitedPartnershipsEmployingHedgeFundStrategiesFocusedOnFixedIncomeAndEquityInvestments**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of total carrying value of limited partnerships reported on a current basis
+ References
No definition available.
+ Details**Name:**cna_PercentageOfLimitedPartnershipsReportedOnCurrentBasis**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of total carrying value of limited partnerships reported on a one month lag.
+ References
No definition available.
+ Details**Name:**cna_PercentageOfLimitedPartnershipsReportedOnOneMonthLag**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Multistrategy Approach Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfMultistrategyApproachLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41678-113959Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226016-175313
+ Details**Name:**us-gaap_DerivativeCollateralObligationToReturnCash**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226016-175313Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41678-113959
+ Details**Name:**us-gaap_DerivativeCollateralRightToReclaimCash**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A valuation allowance for financing receivables that are expected to be uncollectible.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLosses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded Investment in financing receivables that are past due at the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_InvestmentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of consolidated retained earnings that represent undistributed (not yet received) earnings from 50% or less owned persons accounted for by the equity method (equity method investees).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 852 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6462270&loc=d3e57205-112772
+ Details**Name:**us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |