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Investments (Narrative) (Details)
£ in Millions
3 Months Ended
Mar. 31, 2021
USD ($)
investment
shares
Mar. 31, 2020
USD ($)
investment
Mar. 31, 2021
GBP (£)
investment
shares
Dec. 31, 2020
USD ($)
shares
Schedule of Investments [Line Items]        
Money Market Investments $ 175,200,000     $ 268,800,000
Marketable equity securities 700,352,000     573,102,000
Payments to Acquire Marketable Securities 48,036,000 $ 0    
Cumulative realized gain (loss) on marketable securities 0 (2,000,000.0)    
Proceeds from sales of marketable equity securities 0 48,016,000    
Cumulative undistributed income in investments in affiliates 39,400,000     26,100,000
Equity Method Investment Impairment   $ 3,600,000    
Number of Equity Method Investments Impaired | investment   2    
Cost method investments 38,600,000     $ 35,700,000
Gain on a cost method investment 2,723,000 $ 0    
Impairment of cost method investments $ 0 2,577,000    
Concentration in single marketable equity security [Member]        
Schedule of Investments [Line Items]        
Number of marketable equity securities greater than 5% of total assets | investment 0   0  
Concentration in single marketable equity security [Member] | Assets, Total [Member]        
Schedule of Investments [Line Items]        
Minimum percentage of plan assets considered as significant concentrations in pension plans 5.00%      
Markel Corporation [Member]        
Schedule of Investments [Line Items]        
Number Of Shares Held In Investment | shares 44,430   44,430 28,000
Marketable equity securities $ 50,600,000     $ 28,900,000
Intersection Holdings [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 12.00%   12.00%  
Framebridge [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment Impairment   2,700,000    
Graham Healthcare Group [Member]        
Schedule of Investments [Line Items]        
Revenue from Related Parties $ 2,500,000 $ 2,500,000    
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 42.50%   42.50%  
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%  
KIHL [Member] | York Joint Venture [Member]        
Schedule of Investments [Line Items]        
Equity Method Investment, Ownership Percentage 45.00%   45.00%  
Advances to Affiliate | £     £ 22.0  
Loan Receivable, Payment Terms 25 years      
Loan Receivable Fixed Interest Rate 7.00%