| **Consolidated Statements of Stockholders' Equity (USD $)In Millions**    | Total                                           | Common stock [Member]                                                                             | Additional paid-in capital [Member]                                                                             | Retained earnings [Member]                                                                                   | Accumulated other comprehensive income (loss) [Member]                                                                                    | Treasury stock [Member]                                                                              | Notes receivable for the issuance of common stock [Member]                                                                                     |
|---------------------------------------------------------------------------|-------------------------------------------------|---------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------|
| Balance at Dec. 31, 2013                                                  |                                            | $ 683us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember | $ 2,145us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AdditionalPaidInCapitalMember | $ 9,495us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember     | $ 442us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherComprehensiveIncomeMember                 | $ (91)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_TreasuryStockMember | $ (23)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= cna_NotesReceivableForIssuanceOfCommonStockMember                     |
| Net income (loss)                                                         | 13us-gaap_NetIncomeLoss                         |                                                                                              |                                                                                                            | 13us-gaap_NetIncomeLoss/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember               |                                                                                                                                      |                                                                                                 |                                                                                                                                           |
| Other comprehensive income (loss)                                         | 250us-gaap_OtherComprehensiveIncomeLossNetOfTax |                                                                                              |                                                                                                            |                                                                                                         | 250us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherComprehensiveIncomeMember |                                                                                                 |                                                                                                                                           |
| Dividends paid to common stockholders                                     |                                            |                                                                                              |                                                                                                            | (338)us-gaap_DividendsCommonStockCash/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember |                                                                                                                                      |                                                                                                 |                                                                                                                                           |
| Stock-based compensation                                                  |                                            |                                                                                              | (1)cna_StockBasedCompensation/ us-gaap_StatementEquityComponentsAxis= us-gaap_AdditionalPaidInCapitalMember     |                                                                                                         |                                                                                                                                      | 6cna_StockBasedCompensation/ us-gaap_StatementEquityComponentsAxis= us-gaap_TreasuryStockMember      |                                                                                                                                           |
| Decrease (increase) in notes receivable from the issuance of common stock |                                            |                                                                                              |                                                                                                            |                                                                                                         |                                                                                                                                      |                                                                                                 | 1cna_Decreaseincreaseinnotesreceivableforcommonstock/ us-gaap_StatementEquityComponentsAxis= cna_NotesReceivableForIssuanceOfCommonStockMember |
| Balance at Mar. 31, 2014                                                  | 12,582us-gaap_StockholdersEquity                | 683us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember   | 2,144us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AdditionalPaidInCapitalMember   | 9,170us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember       | 692us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherComprehensiveIncomeMember                   | (85)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_TreasuryStockMember   | (22)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= cna_NotesReceivableForIssuanceOfCommonStockMember                       |
| Balance at Dec. 31, 2014                                                  | 12,794us-gaap_StockholdersEquity                | 683us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember   | 2,151us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AdditionalPaidInCapitalMember   | 9,645us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember       | 400us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherComprehensiveIncomeMember                   | (84)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_TreasuryStockMember   | (1)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= cna_NotesReceivableForIssuanceOfCommonStockMember                        |
| Net income (loss)                                                         | 233us-gaap_NetIncomeLoss                        |                                                                                              |                                                                                                            | 233us-gaap_NetIncomeLoss/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember              |                                                                                                                                      |                                                                                                 |                                                                                                                                           |
| Other comprehensive income (loss)                                         | 21us-gaap_OtherComprehensiveIncomeLossNetOfTax  |                                                                                              |                                                                                                            |                                                                                                         | 21us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherComprehensiveIncomeMember  |                                                                                                 |                                                                                                                                           |
| Dividends paid to common stockholders                                     |                                            |                                                                                              |                                                                                                            | (608)us-gaap_DividendsCommonStockCash/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember |                                                                                                                                      |                                                                                                 |                                                                                                                                           |
| Stock-based compensation                                                  |                                            |                                                                                              | (8)cna_StockBasedCompensation/ us-gaap_StatementEquityComponentsAxis= us-gaap_AdditionalPaidInCapitalMember     |                                                                                                         |                                                                                                                                      | 5cna_StockBasedCompensation/ us-gaap_StatementEquityComponentsAxis= us-gaap_TreasuryStockMember      |                                                                                                                                           |
| Decrease (increase) in notes receivable from the issuance of common stock |                                            |                                                                                              |                                                                                                            |                                                                                                         |                                                                                                                                      |                                                                                                 | 0cna_Decreaseincreaseinnotesreceivableforcommonstock/ us-gaap_StatementEquityComponentsAxis= cna_NotesReceivableForIssuanceOfCommonStockMember |
| Balance at Mar. 31, 2015                                                  | $ 12,437us-gaap_StockholdersEquity              | $ 683us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember | $ 2,143us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AdditionalPaidInCapitalMember | $ 9,270us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_RetainedEarningsMember     | $ 421us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherComprehensiveIncomeMember                 | $ (79)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= us-gaap_TreasuryStockMember | $ (1)us-gaap_StockholdersEquity/ us-gaap_StatementEquityComponentsAxis= cna_NotesReceivableForIssuanceOfCommonStockMember                      |

| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Change in equity related to changes in Notes Receivable for Common Stock
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Stock-based compensation includes compensation expense as well as vesting and exercising of shares
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax and reclassification adjustments of other comprehensive income (loss).
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
+ References
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