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Pension and Postretirement Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member]
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2024
USD ($)
investment
company
Mar. 31, 2023
USD ($)
Dec. 31, 2023
USD ($)
investment
Defined Benefit Plan Disclosure [Line Items]      
Special separation benefit expense $ 418 $ 4,129  
Percent of plan assets managed internally by the company 49.00%    
Percent of plan assets managed by investment companies 51.00%    
Number of investment companies actively managing plan assets | company 2    
Percentage of total plan assets 100.00%   100.00%
Markel Corporation [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Value of investments $ 88,300   $ 82,400
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 35.00%    
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%    
Fixed income securities [Member] | Minimum [Member] | Investment Manager 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, target allocation percentage of assets 5.00%    
Private investment fund [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of total plan assets 16.00%   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 $ 1,665,400   $ 1,011,100
Percentage of total plan assets 51.00%   34.00%
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
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
Single Equity Concentration [Member] | US Treasury 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    
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%
Leaf Group and Code3 [Member] | Separation Incentive Program [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Special separation benefit expense   $ 4,100  
Framebridge [Member] | Separation Incentive Program [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Special separation benefit expense $ 400