v3.26.3
Investments (Narrative) (Details)
£ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Sep. 30, 2025
USD ($)
May 31, 2024
USD ($)
Apr. 30, 2023
USD ($)
Sep. 30, 2026
USD ($)
shares
Jun. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
Sep. 30, 2026
USD ($)
shares
Sep. 30, 2025
USD ($)
Sep. 30, 2026
GBP (£)
shares
Dec. 31, 2025
USD ($)
shares
Schedule of Investments [Line Items]                    
Money Market Investments       $ 7,900,000     $ 7,900,000     $ 5,300,000
Marketable equity securities       1,069,741,000     1,069,741,000     1,081,938,000
Payments to Acquire Marketable Securities             25,020,000 $ 29,823,000    
Cumulative realized net gains on sales of marketable securities             28,000,000.0      
Proceeds from sales of marketable equity securities             54,642,000 0    
Cumulative realized gain on marketable securities donated             500,000 400,000    
Impairment on equity method investment       500,000     6,300,000      
Gain in equity earnings       (3,590,000)   $ 15,780,000 11,397,000 10,466,000    
Cumulative undistributed income in investments in affiliates       24,900,000     24,900,000     30,000,000.0
Cost method investments       49,300,000     49,300,000     $ 49,400,000
Impairment of cost method investments       100,000   1,486,000 $ 100,000 14,165,000    
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       $ 4,400,000   $ 4,200,000 $ 12,800,000 $ 12,100,000    
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       $ 109,200,000     $ 109,200,000     $ 119,200,000
Berkshire Hathaway [Member]                    
Schedule of Investments [Line Items]                    
Marketable equity securities       $ 555,200,000     $ 555,200,000      
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       50.60%     50.60%   50.60%  
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       41.50%     41.50%   41.50%  
Impairment on equity method investment         $ 5,800,000          
Intersection [Member]                    
Schedule of Investments [Line Items]                    
Equity Method Investment, Diluted Ownership Percentage       25.60%     25.60%   25.60%  
Payments for Advance to Affiliate     $ 30,000,000              
Additional Investment in Equity Method Investments $ 29,300,000                  
Settlement of Advance To Affiliate 19,300,000                  
Gain in equity earnings $ 18,600,000                  
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 | £                 £ 18.2