|**Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions**                                                                                                                                                                                                                                                                                                                                                                                                            | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | $ 3,304                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | $ 2,926                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Total operating revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 3,336                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 2,985                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Net investment gains (losses)                                                                                                                                                                                                                                                                                                                                                                                                                                                                     | (32)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | (59)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 1,361                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 1,286                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 1,291                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 1,091                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 328                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 284                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Operating Segments | Life & Group                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 342                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 320                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Operating Segments | Corporate & Other                                                                                                                                                                                                                                                                                                                                                                                                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 16                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | 5                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Eliminations                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | (2)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | (1)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Management & Professional Liability | Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 718                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 680                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Surety | Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 183                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 162                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Warranty & Alternative Risks | Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 460                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 444                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Middle Market | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 419                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 374                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Construction | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 420                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 350                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Small Business | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 160                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 143                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Other Commercial | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 292                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 224                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Canada | Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 96                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | 91                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Europe | Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 135                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 118                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Hardy | Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | $ 97                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | $ 75                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|[1]
As of January 1, 2023, the Company adopted  ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts(ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021.  Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance.  See Note A to the Condensed Consolidated Financial Statements for additional information. | [1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | As of January 1, 2023, the Company adopted  ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts(ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021.  Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance.  See Note A to the Condensed Consolidated Financial Statements for additional information.
|[1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | As of January 1, 2023, the Company adopted  ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts(ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021.  Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance.  See Note A to the Condensed Consolidated Financial Statements for additional information.

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