v3.25.2
Pension and Postretirement Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member]
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2025
USD ($)
investment
company
Mar. 31, 2025
USD ($)
Jun. 30, 2024
USD ($)
Mar. 31, 2024
USD ($)
Jun. 30, 2025
USD ($)
investment
company
Jun. 30, 2024
USD ($)
Dec. 31, 2024
USD ($)
investment
Defined Benefit Plan Disclosure [Line Items]              
Special separation benefit expense $ 6,015   $ 16,410   $ 6,639 $ 16,828  
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%
Markel Corporation [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Value of investments $ 115,900       $ 115,900   $ 100,100
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 17.00%       17.00%   19.00%
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 $ 1,215,700       $ 1,215,700   $ 1,178,500
Percentage of total plan assets 37.00%       37.00%   40.00%
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%
Framebridge [Member] | Separation Incentive Program [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Special separation benefit expense       $ 400      
Graham Media Group and Corporate [Member] | VoluntaryRetirementIncentiveProgram [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Special separation benefit expense     14,800        
Framebridge and Code3 [Member] | Separation Incentive Program [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Special separation benefit expense     $ 1,600        
World of Good Brands [Member] | Separation Incentive Program [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Special separation benefit expense   $ 600          
Kaplan, Graham Media Group, WGB, Saatchi Art, Society6, Code3 and Decile [Member] | Separation Incentive Program [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Special separation benefit expense $ 6,000