|**Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions**                                                                                                                                                                                                                                                                                                                                                                                                           | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | $ 3,336                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | $ 2,957                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Total operating revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 3,374                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 3,053                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Net investment gains (losses)                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | (38)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | (96)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 1,372                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 1,310                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 1,335                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 1,148                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 321                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 286                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Operating Segments | Life & Group                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 328                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 305                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Operating Segments | Corporate & Other                                                                                                                                                                                                                                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 21                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Eliminations                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | (3)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | (2)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Management & Professional Liability | Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 723                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 688                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Surety | Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 189                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 171                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Warranty & Alternative Risks | Operating Segments | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 460                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 451                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Middle Market | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 437                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 397                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Construction | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 444                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 372                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Small Business | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 160                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 149                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Other Commercial | Operating Segments | Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 294                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 230                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Canada | Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 98                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 93                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Europe | Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | 139                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 112                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Hardy | Operating Segments | International                                                                                                                                                                                                                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Total revenues                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | $ 84                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | $ 81                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|[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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