XML 63 R46.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Anticipated amounts due from insureds related to losses under deductible policies | $ 1,200 | $ 1,200 |
Structured settlement annuities, interest rate, low end | 5.50% | 5.50% |
Structured settlement annuities, interest rate, high end | 8.00% | 8.00% |
Discounted reserves for unfunded structured settlements | $ 527 | $ 544 |
Discounted reserves for unfunded structure settlements, discount amount | 798 | 841 |
Amount of interest recognized on the discounted reserves of unfunded structured settlements | $ 41 | $ 42 | $ 42
Interest rates at which workers' compensation lifetime claim reserves are discounted at | 3.50% | 3.50% |
Workers' compensation liability | $ 346 | $ 371 |
Discounted reserves for workers’ compensation lifetime claim reserves, discount amount | 190 | 202 |
Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves | $ 19 | $ 17 | 20
Long term care claim reserves discounted interest rate | 6.00% |  |
Minimum interest rate used to discount long term care claim reserves |  | 4.50% |
Maximum interest rate used to discount long term care claim reserves |  | 6.80% |
Discounted reserves for long term care claim reserves | $ 2,400 | $ 2,200 |
Discounted reserves for long term care claim reserves, discount amount | $ 446 | 529 |
Long term care premium deficiency |  |  | 296
Write off of deferred acquisition cost asset due to long term care premium deficiency |  |  | 289
Loss on life settlement contracts, held for sale |  | 10 |
Life settlement contracts, fair value |  | 58 |
Adjustment to Active life reserves due to premium deficiency |  | $ 7 |
Minimum interest rate used to calculate reserves for long term care products | 6.60% | 6.60% |
Maximum interest rate used to calculate reserves for long term care products | 7.00% | 7.00% |
Liability balance for guaranty fund | $ 121 | $ 125 |
Percentage of billed receivables compared to total reinsurance receivable | 3.00% |  |
Deposit assets | $ 8 | 3 |
Deposit liabilities | 4 | 6 |
Increase in fair value recognized on life settlement contracts |  | 7 | 1
Gain recognized on matured life settlement contracts | 3 | 8 | 24
Decreased net investment income and amortized cost of fixed maturity securities due to a change in estimate effected by a change in accounting principle |  |  | 39
Decreased net investment income and amortized cost of fixed maturity securities due to a change in estimate effected by a change in accounting principle relating to 2014 and prior |  |  | $ 22
Decrease of basic and diluted earnings per Share due to change in estimate effected by a change in accounting principle |  |  | $ 0.09
Decrease in shadow adjustment, net of tax | 397 | 97 |
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments | 1,411 | 1,014 |
Foreign currency transaction gain (loss), before tax | $ 27 | $ (9) | $ (11)
Weighted average number diluted shares outstanding adjustment | 988 | 678 | 534
Antidilutive securities excluded from computation of earnings per share, amount | 1 | 102 | 106
Interest paid, net | $ 155 | $ 157 | $ 152
Income taxes paid | $ 152 | $ 170 | $ 310
Furniture and Fixtures |  |  |
Property, plant and equipment, useful life | 7 years |  |
Office Equipment |  |  |
Property, plant and equipment, useful life | 5 years |  |
Computer Software, Intangible Asset |  |  |
Property, plant and equipment, useful life | 10 years |  |
Building and Building Improvements |  |  |
Property, plant and equipment, useful life | 50 years |  |
Minimum |  |  |
Requisite service period for stock- based compensation expense | 3 years |  |
Minimum | Computer Software, Intangible Asset |  |  |
Property, plant and equipment, useful life | 3 years |  |
Maximum |  |  |
Requisite service period for stock- based compensation expense | 4 years |  |
Maximum | Computer Software, Intangible Asset |  |  |
Property, plant and equipment, useful life | 5 years |  |
CNAF Consolidated | Loews |  |  |
Noncontrolling interest, ownership percentage by parent | 89.00% |  |
ASU 2016-09 |  |  |
Increase (decrease) in income tax expense | $ 6 |  |
ASU 2014-09 |  |  |
Increase in other assets and other liabilities | 1,700 |  |
Increase in other revenues and other operating expenses | 500 |  |
ASU 2014-09 | Retained Earnings |  |  |
Cumulative effect of new accounting principle in period of adoption | 65 |  |
ASU 2016-01 | Retained Earnings |  |  |
Cumulative effect of new accounting principle in period of adoption | 28 |  |
ASU 2016-01 | Accumulated Other Comprehensive Income (Loss) |  |  |
Cumulative effect of new accounting principle in period of adoption | 28 |  |
Other assets |  |  |
Note receivable | $ 46 |  |

X
- Definition Adjustment to Active life reserves due to premium deficiency + References No definition available. + Details
Name: | cna_AdjustmenttoActivelifereservesduetopremiumdeficiency
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves + References No definition available. + Details
Name: | cna_Amountofinterestaccretionrecognizedonthediscountedreservesofworkerscompensationlifetimeclaimreserves
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of interest recognized on the discounted reserves of unfunded structured settlements + References No definition available. + Details
Name: | cna_Amountofinterestrecognizedonthediscountedreservesofunfundedstructuredsettlements
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Losses under deductible policies + References No definition available. + Details
Name: | cna_AnticipatedAmountsDueFromInsuredsRelatedToLossesUnderDeductiblePolicies
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Decreased Net Investment Income and amortized cost of fixed maturity securities due to a change in estimate effected by a change in accounting principle + References No definition available. + Details
Name: | cna_DecreasedNetInvestmentIncomeandamortizedcostoffixedmaturitysecuritiesduetoachangeinestimateeffectedbyachangeinaccountingprinciple
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Decreased Net Investment Income and amortized cost of fixed maturity securities due to a change in estimate effected by a change in accounting principle relating to 2014 and prior + References No definition available. + Details
Name: | cna_DecreasedNetInvestmentIncomeandamortizedcostoffixedmaturitysecuritiesduetoachangeinestimateeffectedbyachangeinaccountingprinciplerelatingto2014andprior
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Decrease in shadow adjustment, net of tax + References No definition available. + Details
Name: | cna_Decreaseinshadowadjustmentnetoftax
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Decrease of Basic and Diluted Earnings per Share due to change in estimate effected by a change in accounting principle + References No definition available. + Details
Name: | cna_DecreaseofBasicandDilutedEarningsperShareduetochangeinestimateeffectedbyachangeinaccountingprinciple
Namespace Prefix: | cna_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Discounted reserves for unfunded structure settlements, discount amount + References No definition available. + Details
Name: | cna_DiscountedReservesForUnfundedStructureSettlementsDiscountAmount
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Discounted reserves for unfunded structured settlements + References No definition available. + Details
Name: | cna_DiscountedReservesForUnfundedStructuredSettlements
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Discounted reserves for long term care claim reserves + References No definition available. + Details
Name: | cna_Discountedreservesforlongtermcareclaimreserves
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Discounted reserves for long term care claim reserves, discount amount + References No definition available. + Details
Name: | cna_Discountedreservesforlongtermcareclaimreservesdiscountamount
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Increase in Other Assets and Other Liabilities + References No definition available. + Details
Name: | cna_IncreaseinOtherAssetsandOtherLiabilities
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Increase in Other Revenues and Other Operating Expenses + References No definition available. + Details
Name: | cna_IncreaseinOtherRevenuesandOtherOperatingExpenses
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Liability for long-term care products, interest rate, year of issue, maximum + References No definition available. + Details
Name: | cna_LiabilityForLongTermCareProductsInterestRateYearOfIssueMaximum
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Liability for long-term care products, interest rate, year of issue minimum + References No definition available. + Details
Name: | cna_LiabilityForLongTermCareProductsInterestRateYearOfIssueMinimum
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Long term Care Claim Reserves Discounted Interest Rate + References No definition available. + Details
Name: | cna_LongtermCareClaimReservesDiscountedInterestRate
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum interest rate used to discount on long term care claim reserves. + References No definition available. + Details
Name: | cna_LongtermcareClaimReservesAreDiscountedAtInterestRatesMaximum
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum interest rate used to discount long term care claim reserves. + References No definition available. + Details
Name: | cna_LongtermcareClaimReservesAreDiscountedAtInterestRatesMinimum
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Long term care premium deficiency + References No definition available. + Details
Name: | cna_Longtermcarepremiumdeficiency
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Loss on life settlement contracts, held for sale + References No definition available. + Details
Name: | cna_Lossonlifesettlementcontractsheldforsale
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Percentage Of Billed Receivables Compared To Total Reinsurance Receivable + References No definition available. + Details
Name: | cna_PercentageOfBilledReceivablesComparedToTotalReinsuranceReceivable
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments + References No definition available. + Details
Name: | cna_ReductionOfNetUnrealizedGainsOnInvestmentsIncludedInAociDueToShadowAdjustments
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Maximum interest rate in the range of interest rates used to calculate future benefits to be paid to or on behalf of policyholders and related expenses less the present value of future net premiums using assumptions such as estimates of expected investment yields, mortality, morbidity, terminations and expenses, applicable at the time the insurance contracts are made and cost of settling insured claims and cost incurred in the claim settlement process of structured settlement annuities. + References No definition available. + Details
Name: | cna_StructuredSettlementAnnuitiesInterestRatesHighEnd
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Structured Settlement Annuities, Interest Rates, Low End + References No definition available. + Details
Name: | cna_StructuredSettlementAnnuitiesInterestRatesLowEnd
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Write off of deferred acquisition cost asset due to long term care premium deficiency + References No definition available. + Details
Name: | cna_Writeoffofdeferredacquisitioncostassetduetolongtermcarepremiumdeficiency
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date, net of allowance for doubtful accounts, of account and note receivables due from other than related parties. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3-4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AccountsAndNotesReceivableNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
Name: | us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption. + References No definition available. + Details
Name: | us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Carrying amount of assets as of the balance sheet date pertaining to amounts paid by the insured (including a ceding company) under insurance or reinsurance contracts for which insurance risk is not transferred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6387650&loc=d3e9420-108338 + Details
Name: | us-gaap_DepositContractsAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount of liabilities as of the balance sheet date pertaining to amounts received by the insurer or reinsurer from the insured (including a ceding company) under insurance or reinsurance contracts for which insurance risk is not transferred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6387650&loc=d3e9420-108338 + Details
Name: | us-gaap_DepositContractsLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30700-110894 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30690-110894 + Details
Name: | us-gaap_ForeignCurrencyTransactionGainLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
Name: | us-gaap_IncomeTaxesPaid
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The increase (decrease) during the reporting period in the amounts payable to taxing authorities for taxes that are based on the reporting entity's earnings, net of amounts receivable from taxing authorities for refunds of overpayments or recoveries of income taxes, and in deferred and other tax liabilities and assets. + References No definition available. + Details
Name: | us-gaap_IncreaseDecreaseInIncomeTaxes
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of cash paid, after deduction of cash paid for capitalized interest, for interest. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586 + Details
Name: | us-gaap_InterestPaidNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Fair value amount of life settlement contracts accounted for under the fair value method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 30 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73718008&loc=d3e42732-111610 + Details
Name: | us-gaap_LifeSettlementContractsFairValueMethodCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the gains or losses recognized and realized during the period on investments in life settlement contracts sold during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73718008&loc=d3e42754-111610 + Details
Name: | us-gaap_LifeSettlementContractsFairValueMethodGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments in life settlement contracts held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73718008&loc=d3e42754-111610 + Details
Name: | us-gaap_LifeSettlementContractsFairValueMethodUnrealizedGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The liability as of the balance sheet date representing required funding mandated by statute or regulatory authority that is related directly or indirectly to underwriting activities, including pools for self-insurance and excluding premium taxes and income taxes. The funds derived from the assessments generally enable a government guarantor to pay insurance claims in the event that the underwriting insurer is insolvent or otherwise unable to meet its payment obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 405 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=51828112&loc=d3e23169-109297 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 405 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392405&loc=d3e24119-109301 + Details
Name: | us-gaap_LossContingencyUndiscountedAmountOfInsuranceRelatedAssessmentLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
Name: | us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
Name: | us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Current year monetary amount of the change in worker's compensation reserve liability resulting from the discount applied to reduce the reserve to present value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),13(b)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_WorkersCompensationDiscountAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Percent discount applied to worker's compensation reserve liability to reduce the reserve to present value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14931-158439 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(b)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14754-158437 + Details
Name: | us-gaap_WorkersCompensationDiscountPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Carrying value as of the balance sheet date of obligations and payables pertaining to claims incurred of a workers compensation nature. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(15)(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(20)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(24)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(15)(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_OfficeEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ComputerSoftwareIntangibleAssetMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingAndBuildingImprovementsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_OwnershipAxis=cna_CNAFConsolidatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=cna_LoewsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201609Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201409Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201601Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |