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Investments (Narrative) (Details)
£ in Millions
3 Months Ended
Mar. 31, 2023
USD ($)
shares
Mar. 31, 2022
USD ($)
Mar. 31, 2023
GBP (£)
shares
Dec. 31, 2022
USD ($)
shares
Schedule of Investments [Line Items]        
Money Market Investments $ 2,200,000     $ 7,700,000
Marketable equity securities 603,543,000     609,921,000
Investments in Marketable Securities 4,600,000      
Payments to Acquire Marketable Securities 6,162,000 $ 0    
Cumulative realized (loss) gain on marketable securities (12,200,000) 41,600,000    
Proceeds from sales of marketable equity securities 29,028,000 55,731,000    
Gain on sale of investments in affiliates 15,000 604,000    
Cumulative undistributed income in investments in affiliates 50,100,000     49,100,000
Cost method investments 69,400,000     $ 66,700,000
Gain on a cost method investment $ 1,831,000 0    
Equity Securities [Member] | 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%      
Markel Corporation [Member]        
Schedule of Investments [Line Items]        
Number of Shares Held in Investment | shares 55,430   55,430 55,430
Marketable equity securities $ 70,800,000     $ 73,000,000
Berkshire Hathaway [Member]        
Schedule of Investments [Line Items]        
Marketable equity securities $ 345,600,000      
Berkshire Hathaway [Member] | Common Class A [Member]        
Schedule of Investments [Line Items]        
Number of Shares Held in Investment | shares 422   422  
Berkshire Hathaway [Member] | Common Class B [Member]        
Schedule of Investments [Line Items]        
Number of Shares Held in Investment | shares 482,945   482,945  
N2K Networks [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Diluted Ownership Percentage 49.90%   49.90%  
Intersection Holdings [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 18.00%   18.00%  
Graham Healthcare Group [Member]        
Schedule of Investments [Line Items]        
Revenue from Related Parties $ 3,500,000 3,300,000    
Graham Healthcare Group [Member] | Residential Home Health Illinois [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 40.00%   40.00%  
Graham Healthcare Group [Member] | Residential Hospice Illinois [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 40.00%   40.00%  
Gain on sale of investments in affiliates   600,000    
Graham Healthcare Group [Member] | Residential And Michigan Hospital Joint Venture [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 40.00%   40.00%  
Graham Healthcare Group [Member] | Celtic Healthcare Allegheny Health Network Joint Venture [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 40.00%   40.00%  
Graham Healthcare Group [Member] | Residential Home Health Illinois and Residential Hospice Illinois [Member]        
Schedule of Investments [Line Items]        
Additional investment in equity affiliates   $ 18,500,000    
KIHL [Member] | York Joint Venture [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 45.00%   45.00%  
Cumulative payments for advance to affiliate | £     £ 22.0  
Loan Receivable, Payment Terms 25 years      
Loan Receivable Fixed Interest Rate 7.00%      
Advances to Affiliate | £     £ 20.4