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Investments (Narrative) (Details)
$ in Thousands, £ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
investment
Mar. 31, 2020
USD ($)
investment
Mar. 31, 2019
USD ($)
Dec. 31, 2021
USD ($)
investment
shares
Dec. 31, 2020
USD ($)
investment
shares
Dec. 31, 2019
USD ($)
Dec. 31, 2021
GBP (£)
investment
shares
Schedule of Investments [Line Items]              
Money market investments       $ 0 $ 268,800    
Marketable equity securities       809,997 573,102    
Payments to acquire marketable securities       48,036 20,004 $ 7,499  
Cumulative realized gain on marketable securities       46,000 23,000 9,500  
Proceeds from sales of marketable equity securities       65,499 93,775 19,303  
Cumulative undistributed income in investments in affiliates       52,500 26,100    
Equity Method Investment Impairment $ 6,600 $ 3,600   6,600 $ 3,600    
Number of Equity Method Investments Impaired | investment 1 2     2    
Gain on sale of an equity affiliate       0 $ 1,370 28,994  
Investments in Affiliates       155,444 155,777    
Cost method investments       48,900 35,700    
Net gain on cost method investments       11,756 4,209 5,080  
Impairment of a cost method investment       0 7,327 0  
Private Equity Fund Investment [Member]              
Schedule of Investments [Line Items]              
Investments in Affiliates       $ 72,800 $ 41,100    
Minimum [Member] | Private Equity Fund Investment [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage       4.00%     4.00%
Maximum [Member] | Private Equity Fund Investment [Member]              
Schedule of Investments [Line Items]              
Equity Method Investment, Ownership Percentage       10.00%     10.00%
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
Equity securities [Member] | Concentration in single marketable equity security [Member] | Total Assets [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       44,430 28,000   44,430
Marketable equity securities       $ 54,800 $ 28,900    
Intersection [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          
Gimlet Media [Member]              
Schedule of Investments [Line Items]              
Gain on sale of an equity affiliate     $ 29,000        
Proceeds from Sale of Equity Method Investments     $ 33,500        
Graham Healthcare Group [Member]              
Schedule of Investments [Line Items]              
Revenue from Related Parties       $ 10,900 $ 9,600 $ 9,300  
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 Term       25 years      
Loan Receivable Fixed Interest Rate       7.00%