|**Leases (Tables)**                   | 12 Months Ended                                                                                                                                                                                                                                                                                              
|------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Leases [Abstract]**                 |                                                                                                                                                                                                                                                                                                              
|Assets and liabilities                | The following table presents operating lease ROU assets and lease liabilities.(In millions)December 31, 2023December 31, 2022Operating lease ROU assets$140$155Operating lease liabilities215220                                                                                                             
|                                      |                                                                                                                                                                                                                                                                                                              
|(In millions)                         | December 31, 2023                                                                                                                                                                                                                                                                                            
|Operating lease ROU assets            | $                                                                                                                                                                                                                                                                                                            
|Operating lease liabilities           | 215                                                                                                                                                                                                                                                                                                          
|Operating lease, liability, maturity  | The following table presents the maturities of operating lease liabilities.(In millions)December 31, 20232024$36202532202630202728202825Thereafter103Total lease payments254Less: Discount(39)Total operating lease liabilities$215                                                                          
|                                      |                                                                                                                                                                                                                                                                                                              
|(In millions)                         | December 31, 2023                                                                                                                                                                                                                                                                                            
|2024                                  | $                                                                                                                                                                                                                                                                                                            
|2025                                  | 32                                                                                                                                                                                                                                                                                                           
|2026                                  | 30                                                                                                                                                                                                                                                                                                           
|2027                                  | 28                                                                                                                                                                                                                                                                                                           
|2028                                  | 25                                                                                                                                                                                                                                                                                                           
|Thereafter                            | 103                                                                                                                                                                                                                                                                                                          
|Total lease payments                  | 254                                                                                                                                                                                                                                                                                                          
|Less: Discount                        | (39)                                                                                                                                                                                                                                                                                                         
|Total operating lease liabilities     | $                                                                                                                                                                                                                                                                                                            
|Lease cost                            | The following table presents the weighted average remaining lease term for operating leases and weighted average discount rate used in calculating operating lease ROU assets.December 31, 2023December 31, 2022Weighted average remaining lease term8.4 years9.2 yearsWeighted average discount rate3.7%3.4%
|                                      |                                                                                                                                                                                                                                                                                                              
|                                      | December 31, 2023                                                                                                                                                                                                                                                                                            
|Weighted average remaining lease term | 8.4 years                                                                                                                                                                                                                                                                                                    
|Weighted average discount rate        | 3.7                                                                                                                                                                                                                                                                                                          

X- DefinitionAssets and Liabilities, Lessee [Table Text Block]
+ ReferencesNo definition available.
+ Details**Name:**
cna_AssetsandLiabilitiesLesseeTableTextBlock**Namespace Prefix:**
cna_**Data Type:**
dtr-types1:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionTabular disclosure of lessee's lease cost. Includes, but is not limited to, interest expense for finance lease, amortization of right-of-use asset for finance lease, operating lease cost, short-term lease cost, variable lease cost and sublease income.
+ ReferencesReference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147478964/842-20-50-4
+ Details**Name:**
us-gaap_LeaseCostTableTextBlock**Namespace Prefix:**
us-gaap_**Data Type:**
dtr-types:textBlockItemType**Balance Type:**
na**Period Type:**
duration X- ReferencesNo definition available.
+ Details**Name:**
us-gaap_LeasesAbstract**Namespace Prefix:**
us-gaap_**Data Type:**
xbrli:stringItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionTabular disclosure of undiscounted cash flows of lessee's operating lease liability. Includes, but is not limited to, reconciliation of undiscounted cash flows to operating lease liability recognized in statement of financial position.
+ ReferencesReference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org//1943274/2147478964/842-20-50-6
+ Details**Name:**
us-gaap_LesseeOperatingLeaseLiabilityMaturityTableTextBlock**Namespace Prefix:**
us-gaap_**Data Type:**
dtr-types:textBlockItemType**Balance Type:**
na**Period Type:**
duration