| **Investments (Net investment income) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Gross investment income                                                                          | $ 614          | $ 409          | $ 1,761 | $ 1,576 |
| Investment expense                                                                               | (13)           | (15)           | (42) | (45) |
| Net investment income                                                                            | 601            | 394            | 1,719 | 1,531 |
| Fixed maturity securities [Member]                                                               |           |           |   |   |
| Gross investment income                                                                          | 507            | 494            | 1,528 | 1,505 |
| Short term investments [Member]                                                                  |           |           |   |   |
| Gross investment income                                                                          | 2              | 2              | 5 | 6 |
| Limited partnership investments [Member]                                                         |           |           |   |   |
| Gross investment income                                                                          | 89             | (93)           | 184 | 32 |
| Equity securities [Member]                                                                       |           |           |   |   |
| Gross investment income                                                                          | 4              | 4              | 10 | 16 |
| Mortgage loans [Member]                                                                          |           |           |   |   |
| Gross investment income                                                                          | 5              | 2              | 13 | 6 |
| Trading portfolio [Member]                                                                       |           |           |   |   |
| Gross investment income                                                                          | 7              | (1)            | 18 | 5 |
| Other [Member]                                                                                   |           |           |   |   |
| Gross investment income                                                                          |      | $ 1            | $ 3 | $ 6 |

| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Income Including Realized Gains Losses on Trading Securities, Before Expenses
+ References
No definition available.
+ Details**Name:**cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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
Total expenses related to the generation of investment income in debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688
+ Details**Name:**us-gaap_InvestmentIncomeInvestmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |