v3.26.1
Investments (Narrative) (Details)
£ in Millions
1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2024
USD ($)
Jun. 30, 2026
USD ($)
shares
Jun. 30, 2025
USD ($)
Jun. 30, 2026
USD ($)
shares
Jun. 30, 2025
USD ($)
Jun. 30, 2026
GBP (£)
shares
Dec. 31, 2025
USD ($)
shares
Schedule of Investments [Line Items]              
Money Market Investments   $ 7,300,000   $ 7,300,000     $ 5,300,000
Marketable equity securities   1,083,092,000   1,083,092,000     1,081,938,000
Payments to Acquire Marketable Securities       18,696,000 $ 4,823,000    
Cumulative realized net gains on sales of marketable securities       45,500,000      
Proceeds from sales of marketable equity securities       49,994,000 0    
Cumulative realized gain on marketable securities donated       500,000      
Impairment on equity method investment   5,800,000   5,800,000      
Cumulative undistributed income in investments in affiliates   28,200,000   28,200,000     30,000,000.0
Cost method investments   49,400,000   49,400,000     $ 49,400,000
Impairment of a cost method investment   0 $ 12,679,000 $ 0 12,679,000    
Concentration in single marketable equity security [Member] | Assets, Total [Member]              
Schedule of Investments [Line Items]              
Minimum percentage of total assets considered as significant concentrations       5.00%      
Related Party [Member] | Graham Healthcare Group [Member]              
Schedule of Investments [Line Items]              
Operating Revenues   $ 4,300,000 $ 4,100,000 $ 8,400,000 $ 7,900,000    
Markel Corporation [Member]              
Schedule of Investments [Line Items]              
Number of Shares Held in Investment | shares   63,793   63,793   63,793 55,430
Marketable equity securities   $ 124,600,000   $ 124,600,000     $ 119,200,000
Berkshire Hathaway [Member]              
Schedule of Investments [Line Items]              
Marketable equity securities   $ 557,200,000   $ 557,200,000      
Berkshire Hathaway [Member] | Common Class A [Member]              
Schedule of Investments [Line Items]              
Number of Shares Held in Investment | shares   422   422   422  
Berkshire Hathaway [Member] | Common Class B [Member]              
Schedule of Investments [Line Items]              
Number of Shares Held in Investment | shares   481,920   481,920   481,920  
Residential Home Health Illinois [Member] | Graham Healthcare Group [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage   40.00%   40.00%   40.00%  
Residential Hospice Illinois [Member] | Graham Healthcare Group [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage   40.00%   40.00%   40.00%  
Mary Free Bed at Home [Member] | Graham Healthcare Group [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage   40.00%   40.00%   40.00%  
Allegheny Health Network Healthcare at Home [Member] | Graham Healthcare Group [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage   40.00%   40.00%   40.00%  
N2K Networks [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Diluted Ownership Percentage   50.40%   50.40%   50.40%  
Payments for Advance to Affiliate $ 2,000,000.0            
Loan Receivable Fixed Interest Rate 12.00%            
Realm [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Diluted Ownership Percentage   41.40%   41.40%   41.40%  
Impairment on equity method investment   $ 5,800,000          
Intersection [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Diluted Ownership Percentage   25.20%   25.20%   25.20%  
York Joint Venture [Member] | KIHL [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage   45.00%   45.00%   45.00%  
Loan Receivable Fixed Interest Rate       7.00%      
Cumulative payments for advance to affiliate | £           £ 22.0  
Loan Receivable, Payment Terms       25 years      
Advances to Affiliate | £           £ 18.8