| **Fair Value (Narrative) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |        |
|--------------------------------------------------------------|----------------|----------------|--------|
||
| **Fair Value Disclosures [Abstract]**                        |           |           |   |   |   |
| Fair Value, Assets, Level 1 to Level 2, Transfers            | $ 0            | $ 0            | $ 0    | $ 0 |   |
| Fair Value, Assets, Level 2 to Level 1, Transfers            | 0              | $ 10           | 0      | $ 10 |   |
| Other Invested Assets Overseas Deposit                       | $ 30           |           | $ 30   |   | $ 27 |

| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Assets, Level 1 to Level 2, Transfers
+ References
No definition available.
+ Details**Name:**cna_FairValueAssetsLevel1toLevel2Transfers**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Assets, Level 2 to Level 1, Transfers
+ References
No definition available.
+ Details**Name:**cna_FairValueAssetsLevel2toLevel1Transfers**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Overseas deposits are excluded from the fair value hierarchy because their fair value is recorded using the net asset value per share (or equivalent) practical expedient, which can be redeemed at net asset value in 90 days or less
+ References
No definition available.
+ Details**Name:**cna_OtherInvestedAssetsOverseasDeposit**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FairValueDisclosuresAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |