| **Business Segments (Income statement information) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|----------------------------------------------------------------------------------------|----------------|----------------|
||
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | $ 2,221        | $ 2,059        | $ 6,435 | $ 6,056 |
| Net investment income                                                                  | 422            | 513            | 1,302 | 1,608 |
| Non-insurance warranty revenue                                                         | 399            | 357            | 1,173 | 1,054 |
| Other revenues                                                                         | 11             | 8              | 24 | 19 |
| Total operating revenues                                                               | 3,053          | 2,937          | 8,934 | 8,737 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 1,658          | 1,627          | 4,683 | 4,666 |
| Policyholders’ dividends                                                         | 7              | 5              | 20 | 18 |
| Amortization of deferred acquisition costs                                             | 383            | 368            | 1,101 | 1,084 |
| Non-insurance warranty expense                                                         | 371            | 330            | 1,092 | 973 |
| Other insurance related expenses                                                       | 297            | 253            | 863 | 779 |
| Other expenses                                                                         | 77             | 62             | 222 | 180 |
| Total claims, benefits and expenses                                                    | 2,793          | 2,645          | 7,981 | 7,700 |
| Core income (loss) before income tax                                              | 260            | 292            | 953 | 1,037 |
| Income tax (expense) benefit on core income (loss)                                     | (47)           | (55)           | (179) | (196) |
| Core income (loss)                                                                | 213            | 237            | 774 | 841 |
| Net investment gains (losses)                                                          | (96)           | 22             | (166) | 117 |
| Income tax (expense) benefit on net investment gains (losses)                          | 11             | (3)            | 38 | (22) |
| Net investment gains (losses), after tax                                               | (85)           | 19             | (128) | 95 |
| Net income (loss)                                                                      | 128            | 256            | 646 | 936 |
| Operating Segments | Specialty                                                         |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 810            | 773            | 2,376 | 2,270 |
| Net investment income                                                                  | 102            | 116            | 305 | 367 |
| Non-insurance warranty revenue                                                         | 399            | 357            | 1,173 | 1,054 |
| Other revenues                                                                         | (1)            | 1              | 0 | 1 |
| Total operating revenues                                                               | 1,310          | 1,247          | 3,854 | 3,692 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 459            | 446            | 1,360 | 1,312 |
| Policyholders’ dividends                                                         | 2              | 0              | 5 | 2 |
| Amortization of deferred acquisition costs                                             | 169            | 165            | 488 | 478 |
| Non-insurance warranty expense                                                         | 371            | 330            | 1,092 | 973 |
| Other insurance related expenses                                                       | 88             | 71             | 250 | 212 |
| Other expenses                                                                         | 15             | 13             | 40 | 36 |
| Total claims, benefits and expenses                                                    | 1,104          | 1,025          | 3,235 | 3,013 |
| Core income (loss) before income tax                                              | 206            | 222            | 619 | 679 |
| Income tax (expense) benefit on core income (loss)                                     | (45)           | (49)           | (134) | (148) |
| Core income (loss)                                                                | 161            | 173            | 485 | 531 |
| Operating Segments | Commercial                                                        |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 1,023          | 893            | 2,901 | 2,629 |
| Net investment income                                                                  | 112            | 141            | 343 | 463 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 13             | 7              | 25 | 17 |
| Total operating revenues                                                               | 1,148          | 1,041          | 3,269 | 3,109 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 733            | 720            | 1,916 | 1,947 |
| Policyholders’ dividends                                                         | 5              | 5              | 15 | 16 |
| Amortization of deferred acquisition costs                                             | 163            | 148            | 467 | 449 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 145            | 125            | 409 | 376 |
| Other expenses                                                                         | 3              | 8              | 21 | 28 |
| Total claims, benefits and expenses                                                    | 1,049          | 1,006          | 2,828 | 2,816 |
| Core income (loss) before income tax                                              | 99             | 35             | 441 | 293 |
| Income tax (expense) benefit on core income (loss)                                     | (19)           | (8)            | (91) | (60) |
| Core income (loss)                                                                | 80             | 27             | 350 | 233 |
| Operating Segments | International                                                     |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 270            | 271            | 803 | 789 |
| Net investment income                                                                  | 16             | 14             | 44 | 42 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 0              | 0              | 0 | 1 |
| Total operating revenues                                                               | 286            | 285            | 847 | 832 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 169            | 171            | 487 | 485 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 51             | 55             | 146 | 157 |
| Non-insurance warranty expense                                                         | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 35             | 32             | 112 | 106 |
| Other expenses                                                                         | 11             | 4              | 25 | (4) |
| Total claims, benefits and expenses                                                    | 266            | 262            | 770 | 744 |
| Core income (loss) before income tax                                              | 20             | 23             | 77 | 88 |
| Income tax (expense) benefit on core income (loss)                                     | (1)            | (6)            | (14) | (21) |
| Core income (loss)                                                                | 19             | 17             | 63 | 67 |
| Operating Segments | Life & Group                                                  |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 118            | 123            | 356 | 369 |
| Net investment income                                                                  | 187            | 240            | 600 | 724 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 0              | (1)            | 0 | (1) |
| Total operating revenues                                                               | 305            | 362            | 956 | 1,092 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 310            | 296            | 884 | 899 |
| 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             | 27             | 89 | 77 |
| Other expenses                                                                         | 2              | 1              | 7 | 5 |
| Total claims, benefits and expenses                                                    | 341            | 324            | 980 | 981 |
| Core income (loss) before income tax                                              | (36)           | 38             | (24) | 111 |
| Income tax (expense) benefit on core income (loss)                                     | 14             | 3              | 31 | 9 |
| Core income (loss)                                                                | (22)           | 41             | 7 | 120 |
| Operating Segments | Corporate & Other                                             |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 0              | 0              | (1) | 0 |
| Net investment income                                                                  | 5              | 2              | 10 | 12 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 1              | 2              | 4 | 5 |
| Total operating revenues                                                               | 6              | 4              | 13 | 17 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | (13)           | (6)            | 36 | 23 |
| 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                                                       | 1              | (1)            | 4 | 9 |
| Other expenses                                                                         | 47             | 37             | 133 | 119 |
| Total claims, benefits and expenses                                                    | 35             | 30             | 173 | 151 |
| Core income (loss) before income tax                                              | (29)           | (26)           | (160) | (134) |
| Income tax (expense) benefit on core income (loss)                                     | 4              | 5              | 29 | 24 |
| Core income (loss)                                                                | (25)           | (21)           | (131) | (110) |
| Eliminations                                                                           |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 0              | (1)            | 0 | (1) |
| Net investment income                                                                  | 0              | 0              | 0 | 0 |
| Non-insurance warranty revenue                                                         | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | (2)            | (1)            | (5) | (4) |
| Total operating revenues                                                               | (2)            | (2)            | (5) | (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                                                       | (1)            | (1)            | (1) | (1) |
| Other expenses                                                                         | (1)            | (1)            | (4) | (4) |
| Total claims, benefits and expenses                                                    | (2)            | (2)            | (5) | (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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5,6,7) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263
+ 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
No definition available.
+ 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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 55 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=124504731&loc=d3e11522-158419Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 45 -Paragraph 2 -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=6480726&loc=d3e6691-158385Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (g)(1) -URI https://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(7)(a)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 2B -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124505477&loc=SL117422543-158416Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI https://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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 18 -URI https://asc.fasb.org/extlink&oid=126929396&loc=SL4613673-111683Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124432515&loc=d3e3550-109257Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126958026&loc=SL5780133-109256Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 11 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124431687&loc=d3e22694-107794Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=124431687&loc=d3e22595-107794Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.13) -URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 11 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=124431687&loc=d3e22694-107794Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=124431687&loc=d3e22583-107794
+ 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 -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4,6) -URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.7) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263
+ 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.4) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263
+ 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=99397103&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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 50 -Section 50 -Paragraph 1 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(6)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263
+ 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 -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -URI https://asc.fasb.org/extlink&oid=120401096&loc=d3e574992-122915Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -URI https://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI https://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
+ 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 -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8906-108599Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 40 -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e9031-108599Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 41 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e9038-108599Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8933-108599Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8933-108599Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126920106&loc=SL49130543-203045Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8736-108599Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8736-108599Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 42 -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e9054-108599Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI https://asc.fasb.org/extlink&oid=126920106&loc=SL49130545-203045Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 924 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 11.L) -URI https://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856
+ 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:** |