v3.26.3
Pension Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member]
$ in Thousands
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2026
USD ($)
numberOfRetireesAndBeneficiaries
Sep. 30, 2026
USD ($)
company
investment
Jun. 30, 2026
USD ($)
Mar. 31, 2026
USD ($)
Sep. 30, 2025
USD ($)
Jun. 30, 2025
USD ($)
Mar. 31, 2025
USD ($)
Sep. 30, 2026
USD ($)
company
investment
Sep. 30, 2025
USD ($)
Dec. 31, 2025
USD ($)
investment
Defined Benefit Plan Disclosure [Line Items]                    
Settlement gain   $ 14,950     $ 0     $ 151,905 $ 0  
Early retirement and separation program costs   $ 3,201     2,546     $ 11,138 $ 9,185  
Percent of plan assets managed internally by the company               41.00%    
Percent of plan assets managed by investment companies               59.00%    
Number of investment companies actively managing plan assets | company   2           2    
Percentage of total plan assets   100.00%           100.00%   100.00%
Irrevocable Group Annuity Contract Purchase [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Payment for settlement $ 113,900                  
Settlement of obligation $ 124,300                  
Number of retirees and beneficiaries | numberOfRetireesAndBeneficiaries 1,080                  
Settlement gain $ 137,000 $ 15,000                
Markel Corporation [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Value of investments   $ 99,300           $ 99,300   $ 124,700
Berkshire Hathaway Common Stock [Member] | Investment Manager 2 [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator               20.00%    
Alphabet and Berkshire Hathaway Common Stock [Member] | Investment Manager 1 [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator               15.00%    
Foreign Investments [Member] | Maximum [Member] | Investment Manager 1 [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Defined benefit plan, target allocation percentage of assets   50.00%           50.00%    
Foreign Investments [Member] | Maximum [Member] | Investment Manager 2 [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Defined benefit plan, target allocation percentage of assets   15.00%           15.00%    
Private investment fund [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Percentage of total plan assets   18.00%           18.00%   17.00%
Single Equity Concentration [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator               10.00%    
Value of investments   $ 1,253,700           $ 1,253,700   $ 1,267,400
Percentage of total plan assets   38.00%           38.00%   37.00%
Single Equity Concentration [Member] | Equity Securities [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment   1           1   1
Single Equity Concentration [Member] | Private investment fund [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment   1           1   1
Concentration In Single Entity, Type Of Industry, Foreign Country Or Individual Fund [Member] | Defined Benefit Plan Assets Total [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Minimum percentage of plan assets considered as significant concentrations in pension plans               10.00%   10.00%
Kaplan, Dekko, Framebridge and Saatchi Art [Member] | Separation Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs   $ 2,400                
Corporate Office [Member] | Voluntary Retirement Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs   $ 800                
Dekko [Member] | Voluntary Retirement Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs     $ 2,000              
Kaplan, Graham Media Group, Code3 and Slate [Member] | Separation Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs     $ 1,900              
Kaplan, Graham Media Group, Joyce, Dekko, Framebridge, Code3, Slate and Corporate [Member] | Separation Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs       $ 4,100            
Kaplan, Graham Media Group, Joyce, Society6, Saatchi Art, Code3 and Corporate [Member] | Separation Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs         $ 2,500          
Kaplan, Graham Media Group, WGB, Saatchi Art, Society6, Code3 and Decile [Member] | Separation Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs           $ 6,000        
World of Good Brands [Member] | Separation Incentive Program [Member]                    
Defined Benefit Plan Disclosure [Line Items]                    
Early retirement and separation program costs             $ 600