| **Benefit Plans (Components of net periodic cost (benefit)) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                                                                                                                |
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| **Defined Benefit Plan Disclosure [Line Items]**                                                                       |                                                                                                                                                                                           |   |   |
| Curtailment gain                                                                                                       | $ (86)cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments                                                                                                                                 |   |   |
| Settlement loss                                                                                                        | 84cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements                                                                                                                                      |   |   |
| Pension Plans, Defined Benefit [Member]                                                                                |                                                                                                                                                                                           |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                       |                                                                                                                                                                                           |   |   |
| Service cost                                                                                                           | 9us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                | 12us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 12us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Interest cost on projected benefit obligation                                                                          | 132us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                             | 121us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 135us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Expected return on plan assets                                                                                         | (191)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                             | (181)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | (171)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Amortization of net actuarial (gain) loss                                                                              | 25us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                 | 47us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 39us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Settlement loss                                                                                                        | 84cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                             | 3cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Net periodic (benefit) cost                                                                                            | 59us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                    | 2us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 15us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Other Postretirement Benefit Plans, Defined Benefit [Member]                                                           |                                                                                                                                                                                           |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                       |                                                                                                                                                                                           |   |   |
| Service cost                                                                                                           | 0us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                             | 1us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 0us-gaap_DefinedBenefitPlanServiceCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Interest cost on projected benefit obligation                                                                          | 1us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                            | 1us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 2us-gaap_DefinedBenefitPlanInterestCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Amortization of prior service credit                                                                                   | (10)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | (18)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | (18)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Amortization of net actuarial (gain) loss                                                                              | 1us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember               | 2us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 1us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Curtailment gain                                                                                                       | (86)cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember       |   |   |
| Net periodic (benefit) cost                                                                                            | $ (94)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember             | $ (14)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | $ (15)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |

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| - Definition
The amount of net gain recognized in net periodic benefit cost as a result of an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services.
+ References
No definition available.
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| - Definition
Amount of net loss recognized in net periodic benefit cost as a result of an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include, but are not limited to, lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contracts to cover vested benefits.
+ References
No definition available.
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| - Definition
The amount of gains or losses recognized in net periodic benefit cost.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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| - Definition
The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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| - Definition
An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512136Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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| - Definition
The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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| - Definition
The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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| - Definition
The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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