XML 75 R65.htm IDEA: XBRL DOCUMENT v3.25.3
Pension and Postretirement Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member]
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2025
USD ($)
investment
company
Jun. 30, 2025
USD ($)
Mar. 31, 2025
USD ($)
Sep. 30, 2024
USD ($)
Jun. 30, 2024
USD ($)
Mar. 31, 2024
USD ($)
Sep. 30, 2025
USD ($)
investment
company
Sep. 30, 2024
USD ($)
Dec. 31, 2024
USD ($)
investment
Defined Benefit Plan Disclosure [Line Items]                  
Special separation benefit expense $ 2,546     $ 3,665     $ 9,185 $ 20,493  
Percent of plan assets managed internally by the company             42.00%    
Percent of plan assets managed by investment companies             58.00%    
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 $ 110,900           $ 110,900   $ 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 18.00%           18.00%   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 $ 1,283,900           $ 1,283,900   $ 1,178,500
Percentage of total plan assets 38.00%           38.00%   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%
Kaplan, Graham Media Group, Joyce, Society6, Saatchi Art, Code3 and Corporate [Member] | Separation Incentive Program [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Special separation benefit expense $ 2,500                
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              
World of Good Brands [Member] | Separation Incentive Program [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Special separation benefit expense     $ 600            
Kaplan, Dekko, WGB, Saatchi Art, Society6, Slate and Decile [Member] | Separation Incentive Program [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Special separation benefit expense       $ 3,700          
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        
Framebridge [Member] | Separation Incentive Program [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Special separation benefit expense           $ 400