| **Stockholders' Equity and Statutory Accounting Practices (Narrative) (Details) - USD ($) $ in Millions**                       | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Stockholders' Equity and Statutory Accounting Practices [Abstract]**                                                          |            |   |
| Increase in statutory capital and surplus due to prescribed practice                                                            | $ 74            | $ 67 |
| Statutory amount available for dividend payment before consideration of statutory dividends paid in the preceding twelve months | 1,057           |   |
| Dividends paid in the preceding twelve months                                                                                   | $ 990           |   |
| Statutory accounting practices, statutory capital and surplus, percentage of action level risk based capital                    | 238.00%         | 264.00% |

| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Statutory Accounting Practices, Dividends Paid in the preceding twelve months
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments Before Consideration of Statutory Dividends Paid in the preceding twelve months
+ References
No definition available.
+ Details**Name:**cna_StatutoryAccountingPracticesStatutoryAmountAvailableforDividendPaymentsBeforeConsiderationofStatutoryDividendsPaidintheprecedingtwelvemonths**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Statutory Accounting Practices, Statutory Capital And Surplus, Percentage Of Action Level Risk Based Capital
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Stockholders' Equity and Statutory Accounting Practices
+ References
No definition available.
+ Details**Name:**cna_StockholdersEquityAndStatutoryAccountingPracticesAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount as of the balance sheet date of each item listed to reconcile the differences resulting from applying state prescribed statutory accounting practices rather than statutory accounting practices prescribed by National Association of Insurance Commissioners.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 3 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=6486098&loc=d3e24746-158529
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