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Debt (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2023
Dec. 31, 2022
Line of Credit Facility [Line Items]    
Line of Credit Facility, Initiation Date Jun. 23, 2023  
Line of Credit Facility, Expiration Date Jun. 23, 2028  
Senior Notes | 3.625% Unsecured Senior Notes due March 30, 2023    
Line of Credit Facility [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 3.625%  
Debt Instrument, Face Amount $ 250.0  
Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity 300.0  
Line of Credit Facility, Increase 200.0  
Long-term Line of Credit 0.0 $ 0.0
Revolving Credit Facility | Markel Ventures [Member]    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity 715.0  
Long-term Line of Credit 209.0 $ 238.1
Revolving Credit Facility | Secured Letters of Credit    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity 200.0  
Previous line of credit facility [Member]    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity $ 300.0