Investments (Narrative) (Details) £ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) shares	Mar. 31, 2023 USD ($)	Mar. 31, 2024 GBP (£) shares	Dec. 31, 2023 USD ($) shares
Schedule of Investments [Line Items]
Money Market Investments	$ 7,500,000			$ 5,600,000
Marketable equity securities	794,305,000			690,153,000
Payments to Acquire Marketable Securities	0	$ 6,162,000
Investments in Marketable Securities		4,600,000
Cumulative realized loss on marketable securities	0	(12,200,000)
Proceeds from sales of marketable equity securities	0	29,028,000
Cumulative undistributed income in investments in affiliates	39,600,000			36,900,000
Cost method investments	74,000,000			$ 74,000,000
Impairment of cost method investments	406,000	0
Gain on a cost method investment	$ 0	1,831,000
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	$ 4,100,000	$ 3,500,000
Markel Corporation [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment | shares	55,430		55,430	55,430
Marketable equity securities	$ 84,300,000			$ 78,700,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities	$ 470,800,000
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%
Realm [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage	42.20%		42.20%
Intersection Holdings [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	18.00%		18.00%
Payments for Advance to Affiliate	$ 30,000,000
Loan Receivable, Payment Terms	5 years
Loan Receivable Fixed Interest Rate	9.00%
Advances to Affiliate	$ 28,300,000
Residential Home Health Illinois [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	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%
Mary Free Bed at Home [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	40.00%		40.00%
Allegheny Health Network (AHN) Healthcare at Home [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	40.00%		40.00%
York Joint Venture [Member] | KIHL [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	45.00%		45.00%
Loan Receivable, Payment Terms	25 years
Loan Receivable Fixed Interest Rate	7.00%
Advances to Affiliate | £			£ 19.9
Cumulative payments for advance to affiliate | £			£ 22.0