| **Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------|----------------|----------------|
||
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | $ 2,498        | $ 2,347        | $ 4,939 | $ 4,595 |
| Net investment income                                                                  | 618            | 575            | 1,227 | 1,100 |
| Non-insurance warranty revenue                                                         | 404            | 407            | 811 | 814 |
| Other revenues                                                                         | 9              | 7              | 18 | 14 |
| Total operating revenues                                                               | 3,529          | 3,336          | 6,995 | 6,523 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 1,874          | 1,772          | 3,672 | 3,418 |
| Policyholders’ dividends                                                         | 8              | 7              | 17 | 14 |
| Amortization of deferred acquisition costs                                             | 435            | 403            | 879 | 782 |
| Non-insurance warranty expense                                                         | 388            | 384            | 782 | 768 |
| Other insurance related expenses                                                       | 329            | 318            | 616 | 623 |
| Other expenses                                                                         | 83             | 59             | 168 | 119 |
| Total claims, benefits and expenses                                                    | 3,117          | 2,943          | 6,134 | 5,724 |
| Core income (loss) before income tax                                              | 412            | 393            | 861 | 799 |
| Income tax (expense) benefit on core income (loss)                                     | (86)           | (85)           | (180) | (166) |
| Core income (loss)                                                                | 326            | 308            | 681 | 633 |
| Net investment gains (losses)                                                          | (10)           | (32)           | (32) | (67) |
| Income tax (expense) benefit on net investment gains (losses)                          | 1              | 7              | 6 | 14 |
| Net investment gains (losses), after tax                                               | (9)            | (25)           | (26) | (53) |
| Net income (loss)                                                                      | 317            | 283            | 655 | 580 |
| Operating Segments | Specialty                                                         |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 831            | 812            | 1,645 | 1,609 |
| Net investment income                                                                  | 154            | 142            | 304 | 271 |
| Non-insurance warranty revenue                                                         | 404            | 407            | 811 | 814 |
| Other revenues                                                                         | (1)            | 0              | 0 | 0 |
| Total operating revenues                                                               | 1,388          | 1,361          | 2,760 | 2,694 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 492            | 474            | 969 | 939 |
| Policyholders’ dividends                                                         | 3              | 2              | 5 | 3 |
| Amortization of deferred acquisition costs                                             | 180            | 168            | 358 | 333 |
| Non-insurance warranty expense                                                         | 388            | 384            | 782 | 768 |
| Other insurance related expenses                                                       | 96             | 94             | 177 | 180 |
| Other expenses                                                                         | 13             | 12             | 28 | 26 |
| Total claims, benefits and expenses                                                    | 1,172          | 1,134          | 2,319 | 2,249 |
| Core income (loss) before income tax                                              | 216            | 227            | 441 | 445 |
| Income tax (expense) benefit on core income (loss)                                     | (47)           | (50)           | (95) | (97) |
| Core income (loss)                                                                | 169            | 177            | 346 | 348 |
| Operating Segments | Commercial                                                        |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 1,247          | 1,120          | 2,449 | 2,166 |
| Net investment income                                                                  | 175            | 165            | 351 | 314 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 10             | 6              | 18 | 13 |
| Total operating revenues                                                               | 1,432          | 1,291          | 2,818 | 2,493 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 846            | 740            | 1,674 | 1,428 |
| Policyholders’ dividends                                                         | 5              | 5              | 12 | 11 |
| Amortization of deferred acquisition costs                                             | 199            | 175            | 399 | 344 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 158            | 158            | 296 | 300 |
| Other expenses                                                                         | 13             | 10             | 25 | 16 |
| Total claims, benefits and expenses                                                    | 1,221          | 1,088          | 2,406 | 2,099 |
| Core income (loss) before income tax                                              | 211            | 203            | 412 | 394 |
| Income tax (expense) benefit on core income (loss)                                     | (44)           | (44)           | (87) | (84) |
| Core income (loss)                                                                | 167            | 159            | 325 | 310 |
| Operating Segments | International                                                     |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 311            | 302            | 626 | 592 |
| Net investment income                                                                  | 32             | 25             | 63 | 48 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 0              | 1              | 0 | 1 |
| Total operating revenues                                                               | 343            | 328            | 689 | 641 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 184            | 185            | 373 | 374 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 56             | 60             | 122 | 105 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 46             | 35             | 85 | 82 |
| Other expenses                                                                         | 1              | (5)            | 3 | (4) |
| Total claims, benefits and expenses                                                    | 287            | 275            | 583 | 557 |
| Core income (loss) before income tax                                              | 56             | 53             | 106 | 84 |
| Income tax (expense) benefit on core income (loss)                                     | (12)           | (15)           | (25) | (22) |
| Core income (loss)                                                                | 44             | 38             | 81 | 62 |
| Operating Segments | Life & Group                                                  |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 109            | 113            | 219 | 228 |
| Net investment income                                                                  | 239            | 229            | 470 | 443 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 0              | 0              | 0 | 0 |
| Total operating revenues                                                               | 348            | 342            | 689 | 671 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 325            | 344            | 637 | 655 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0              | 0              | 0 | 0 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 29             | 31             | 58 | 60 |
| Other expenses                                                                         | 1              | 0              | 1 | 1 |
| Total claims, benefits and expenses                                                    | 355            | 375            | 696 | 716 |
| Core income (loss) before income tax                                              | (7)            | (33)           | (7) | (45) |
| Income tax (expense) benefit on core income (loss)                                     | 6              | 13             | 11 | 22 |
| Core income (loss)                                                                | (1)            | (20)           | 4 | (23) |
| Operating Segments | Corporate & Other                                             |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 0              | 0              | 0 | 0 |
| Net investment income                                                                  | 18             | 14             | 39 | 24 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 3              | 2              | 6 | 5 |
| Total operating revenues                                                               | 21             | 16             | 45 | 29 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 27             | 29             | 19 | 22 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0              | 0              | 0 | 0 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 0              | 0              | 0 | 1 |
| Other expenses                                                                         | 58             | 44             | 117 | 85 |
| Total claims, benefits and expenses                                                    | 85             | 73             | 136 | 108 |
| Core income (loss) before income tax                                              | (64)           | (57)           | (91) | (79) |
| Income tax (expense) benefit on core income (loss)                                     | 11             | 11             | 16 | 15 |
| Core income (loss)                                                                | (53)           | (46)           | (75) | (64) |
| Eliminations                                                                           |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 0              | 0              | 0 | 0 |
| Net investment income                                                                  | 0              | 0              | 0 | 0 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | (3)            | (2)            | (6) | (5) |
| Total operating revenues                                                               | (3)            | (2)            | (6) | (5) |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 0              | 0              | 0 | 0 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0              | 0              | 0 | 0 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 0              | 0              | 0 | 0 |
| Other expenses                                                                         | (3)            | (2)            | (6) | (5) |
| Total claims, benefits and expenses                                                    | (3)            | (2)            | (6) | (5) |
| Core income (loss) before income tax                                              | 0              | 0              | 0 | 0 |
| Income tax (expense) benefit on core income (loss)                                     | 0              | 0              | 0 | 0 |
| Core income (loss)                                                                | $ 0            | $ 0            | $ 0 | $ 0 |

| X                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Costs From Contract With Customers
+ References
No definition available.
+ Details**Name:**cna_CostsFromContractWithCustomers**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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
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/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(5)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1
+ 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 realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(9)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(7)(c)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(3)(d)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1
+ Details**Name:**us-gaap_DebtSecuritiesRealizedGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480016/944-40-65-2Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 944 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479432/944-30-50-2Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 2B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479432/944-30-50-2BReference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479401/944-30-55-2Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479868/944-20-45-2Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(7)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479432/944-30-50-1
+ 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/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147483443/250-10-50-6Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 805 -SubTopic 60 -Name Accounting Standards Codification -Section 65 -Paragraph 1 -Subparagraph (g) -Publisher FASB -URI https://asc.fasb.org/1943274/2147476176/805-60-65-1Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(13)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147483443/250-10-50-3Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483443/250-10-50-11Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483443/250-10-50-11Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 250 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147483443/250-10-50-4Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482662/260-10-50-1Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 60B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482689/260-10-45-60BReference 10: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481231/810-10-45-18
+ 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://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(4)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2
+ 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/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column E)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column E)(Footnote 4)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column E)(Footnote 6)(b)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(4)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-07(1)(c)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1
+ 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/2009/role/commonPracticeRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(5)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1
+ 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/disclosureRef -Topic 944 -SubTopic 50 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479787/944-50-45-1Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(6)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -SubTopic 50 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479756/944-50-50-1
+ 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/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-2Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479032/944-605-50-1Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -SubTopic 220 -Topic 942 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1
+ Details**Name:**us-gaap_PremiumsEarnedNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 41 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-41Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 270 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482964/270-10-50-1Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 32 -Subparagraph (ee) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-32Reference 4: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 32 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-32Reference 5: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 32 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-32Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 924 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 11.L) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479941/924-10-S99-1Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 606 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479806/606-10-50-5Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 30 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-30Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 42 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-42Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 22 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-22Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 40 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-40Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 22 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-22Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 606 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479806/606-10-50-4
+ Details**Name:**us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax**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                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtySegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_LifeAndGroupNonCoreSegmentMember**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:**srt_ConsolidationItemsAxis=us-gaap_IntersegmentEliminationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |