| **Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions** | 3 Months Ended | 12 Months Ended |
|----------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | $ 7,069         | $ 6,988 | $ 6,962 |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | 6,988           | 6,924 | 6,921 |
| Net investment income                                                                  |           | 2,034           | 1,988 | 1,840 |
| Other revenues                                                                         |           | 427             | 404 | 407 |
| Total operating revenues                                                               |           | 9,449           | 9,316 | 9,168 |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | 5,288           | 5,270 | 5,372 |
| Policyholders’ dividends                                                         |           | 22              | 13 | 12 |
| Amortization of deferred acquisition costs                                             |           | 1,233           | 1,235 | 1,540 |
| Other insurance related expenses                                                       |           | 1,098           | 1,122 | 1,093 |
| Other expenses                                                                         |           | 591             | 589 | 535 |
| Total claims, benefits and expenses                                                    |           | 8,232           | 8,229 | 8,552 |
| Core income (loss) before income tax                                                   |           | 1,217           | 1,087 | 616 |
| Income tax (expense) benefit on core income (loss)                                     |           | (298)           | (263) | (101) |
| Core income (loss)                                                                     |           | 919             | 824 | 515 |
| Net realized investment gains (losses)                                                 |           | 93              | 50 | (67) |
| Income tax (expense) benefit on net realized investment gains (losses)                 |           | (30)            | (15) | 31 |
| Net realized investment gains (losses), after tax                                      |           | 63              | 35 | (36) |
| Net deferred tax asset remeasurement                                                   | $ (83)         | (83)            | 0 | 0 |
| Net income                                                                             |           | 899             | 859 | 479 |
| Operating Segments | Specialty                                                         |           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | 2,771           | 2,780 | 2,781 |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | 2,753           | 2,779 | 2,782 |
| Net investment income                                                                  |           | 538             | 516 | 474 |
| Other revenues                                                                         |           | 392             | 362 | 356 |
| Total operating revenues                                                               |           | 3,683           | 3,657 | 3,612 |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | 1,535           | 1,467 | 1,597 |
| Policyholders’ dividends                                                         |           | 4               | 4 | 4 |
| Amortization of deferred acquisition costs                                             |           | 596             | 591 | 589 |
| Other insurance related expenses                                                       |           | 286             | 301 | 278 |
| Other expenses                                                                         |           | 342             | 312 | 301 |
| Total claims, benefits and expenses                                                    |           | 2,763           | 2,675 | 2,769 |
| Core income (loss) before income tax                                                   |           | 920             | 982 | 843 |
| Income tax (expense) benefit on core income (loss)                                     |           | (310)           | (332) | (283) |
| Core income (loss)                                                                     |           | 610             | 650 | 560 |
| Operating Segments | Commercial                                                        |           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | 2,882           | 2,841 | 2,818 |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | 2,840           | 2,804 | 2,788 |
| Net investment income                                                                  |           | 642             | 638 | 593 |
| Other revenues                                                                         |           | 31              | 32 | 37 |
| Total operating revenues                                                               |           | 3,513           | 3,474 | 3,418 |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | 1,928           | 1,927 | 1,814 |
| Policyholders’ dividends                                                         |           | 18              | 9 | 8 |
| Amortization of deferred acquisition costs                                             |           | 475             | 470 | 469 |
| Other insurance related expenses                                                       |           | 523             | 560 | 538 |
| Other expenses                                                                         |           | 57              | 36 | 28 |
| Total claims, benefits and expenses                                                    |           | 3,001           | 3,002 | 2,857 |
| Core income (loss) before income tax                                                   |           | 512             | 472 | 561 |
| Income tax (expense) benefit on core income (loss)                                     |           | (171)           | (161) | (192) |
| Core income (loss)                                                                     |           | 341             | 311 | 369 |
| Operating Segments | International                                                     |           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | 881             | 821 | 822 |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | 857             | 806 | 804 |
| Net investment income                                                                  |           | 52              | 51 | 52 |
| Other revenues                                                                         |           | 0               | 0 | (1) |
| Total operating revenues                                                               |           | 909             | 857 | 855 |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | 575             | 492 | 479 |
| Policyholders’ dividends                                                         |           | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             |           | 162             | 174 | 168 |
| Other insurance related expenses                                                       |           | 162             | 133 | 138 |
| Other expenses                                                                         |           | (7)             | 24 | 12 |
| Total claims, benefits and expenses                                                    |           | 892             | 823 | 797 |
| Core income (loss) before income tax                                                   |           | 17              | 34 | 58 |
| Income tax (expense) benefit on core income (loss)                                     |           | (9)             | (13) | (21) |
| Core income (loss)                                                                     |           | 8               | 21 | 37 |
| Operating Segments | Life & Group                                                  |           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | 536             | 547 | 542 |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | 539             | 536 | 548 |
| Net investment income                                                                  |           | 782             | 767 | 704 |
| Other revenues                                                                         |           | 2               | (2) | 7 |
| Total operating revenues                                                               |           | 1,323           | 1,301 | 1,259 |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | 1,269           | 1,286 | 1,421 |
| Policyholders’ dividends                                                         |           | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             |           | 0               | 0 | 314 |
| Other insurance related expenses                                                       |           | 129             | 132 | 142 |
| Other expenses                                                                         |           | 7               | 8 | 11 |
| Total claims, benefits and expenses                                                    |           | 1,405           | 1,426 | 1,888 |
| Core income (loss) before income tax                                                   |           | (82)            | (125) | (629) |
| Income tax (expense) benefit on core income (loss)                                     |           | 132             | 145 | 315 |
| Core income (loss)                                                                     |           | 50              | 20 | (314) |
| Operating Segments | Corporate & Other                                             |           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | 0               | 0 | 1 |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | 0               | 0 | 1 |
| Net investment income                                                                  |           | 20              | 16 | 17 |
| Other revenues                                                                         |           | 2               | 12 | 11 |
| Total operating revenues                                                               |           | 22              | 28 | 29 |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | (19)            | 98 | 61 |
| Policyholders’ dividends                                                         |           | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             |           | 0               | 0 | 0 |
| Other insurance related expenses                                                       |           | (1)             | (3) | (1) |
| Other expenses                                                                         |           | 192             | 209 | 186 |
| Total claims, benefits and expenses                                                    |           | 172             | 304 | 246 |
| Core income (loss) before income tax                                                   |           | (150)           | (276) | (217) |
| Income tax (expense) benefit on core income (loss)                                     |           | 60              | 98 | 80 |
| Core income (loss)                                                                     |           | (90)            | (178) | (137) |
| Eliminations                                                                           |           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |           |            |   |   |
| Net written premiums                                                                   |           | (1)             | (1) | (2) |
| **Operating revenues**                                                                 |           |            |   |   |
| Net earned premiums                                                                    |           | (1)             | (1) | (2) |
| Net investment income                                                                  |           | 0               | 0 | 0 |
| Other revenues                                                                         |           | 0               | 0 | (3) |
| Total operating revenues                                                               |           | (1)             | (1) | (5) |
| **Claims, Benefits and Expenses**                                                      |           |            |   |   |
| Net incurred claims and benefits                                                       |           | 0               | 0 | 0 |
| Policyholders’ dividends                                                         |           | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             |           | 0               | 0 | 0 |
| Other insurance related expenses                                                       |           | (1)             | (1) | (2) |
| Other expenses                                                                         |           | 0               | 0 | (3) |
| Total claims, benefits and expenses                                                    |           | (1)             | (1) | (5) |
| Core income (loss) before income tax                                                   |           | 0               | 0 | 0 |
| Income tax (expense) benefit on core income (loss)                                     |           | 0               | 0 | 0 |
| Core income (loss)                                                                     |           | $ 0             | $ 0 | $ 0 |

| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Effective Income Tax Rate Reconciliation, Effect of Tax Cuts and Jobs Act of 2017, Amount
+ References
No definition available.
+ Details**Name:**cna_EffectiveIncomeTaxRateReconciliationEffectofTaxCutsandJobsActof2017Amount**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Net Realized Investment Gains Losses
+ References
No definition available.
+ Details**Name:**cna_IncomeTaxExpenseBenefitOnNetRealizedInvestmentGainsLosses**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Operating Income (Loss)
+ References
No definition available.
+ Details**Name:**cna_IncomeTaxExpenseBenefitOnOperatingIncomeLoss**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It also includes realized gains and losses on trading securities.
+ References
No definition available.
+ Details**Name:**cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Operating Income (Loss) Attributable to Parent
+ References
No definition available.
+ Details**Name:**cna_NetOperatingIncomeLossFromContinuingOperationsAttributabletoParent**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net realized investment gains (losses)
+ References
No definition available.
+ Details**Name:**cna_NetRealizedInvestmentGainsLosses**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Operating Income (Loss) before Income Tax
+ References
No definition available.
+ Details**Name:**cna_OperatingIncomeLossFromContinuingOperationsBeforeIncomeTax**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Operating Revenues
+ References
No definition available.
+ Details**Name:**cna_OperatingRevenues**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Insurance Related Expenses
+ References
No definition available.
+ Details**Name:**cna_OtherInsuranceRelatedExpenses**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5,6,7) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913
+ Details**Name:**us-gaap_BenefitsLossesAndExpenses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_BenefitsLossesAndExpensesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization expense (reversal of expense) for deferred policy acquisition costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(7)(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 35 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=95462302&loc=d3e10157-158409Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column I)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column I)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 45 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6480726&loc=d3e6691-158385Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of income (loss) from continuing operations attributable to the parent.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585Reference 2: 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=d3e3550-109257Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.13) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4613673-111683
+ Details**Name:**us-gaap_IncomeLossFromContinuingOperations**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense classified as other.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.7) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4,6) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688
+ Details**Name:**us-gaap_OtherExpenses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of revenue and income classified as other.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.4) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913
+ Details**Name:**us-gaap_OtherIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387
+ Details**Name:**us-gaap_PolicyholderBenefitsAndClaimsIncurredNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense for dividends to policyholders.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 50 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.6) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913
+ Details**Name:**us-gaap_PolicyholderDividends**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915
+ Details**Name:**us-gaap_PremiumsEarnedNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
+ Details**Name:**us-gaap_PremiumsWrittenNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_RevenuesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| 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.
+ Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_LifeAndGroupNonCoreMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ConsolidationItemsAxis=us-gaap_IntersegmentEliminationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |