Investments (Narrative) (Details) £ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2024 USD ($)	Jun. 30, 2024 USD ($) shares	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) shares	Jun. 30, 2023 USD ($)	Jun. 30, 2024 GBP (£) shares	Dec. 31, 2023 USD ($) shares
Schedule of Investments [Line Items]
Money Market Investments		$ 9,000,000.0		$ 9,000,000.0			$ 5,600,000
Marketable equity securities		813,444,000		813,444,000			690,153,000
Payments to Acquire Marketable Securities				0	$ 6,162,000
Investments in Marketable Securities					4,600,000
Cumulative realized gain on marketable securities sold or donated				400,000	13,000,000.0
Proceeds from sales of marketable equity securities				0	61,979,000
Payments for Advance to Affiliate				2,000,000	30,000,000
Cumulative undistributed income in investments in affiliates		37,600,000		37,600,000			36,900,000
Cost method investments		74,400,000		74,400,000			$ 74,000,000.0
Impairment of cost method investments		338,000	$ 0	744,000	0
Gain on cost method investments		0	1,273,000	$ 0	3,104,000
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	$ 3,900,000	$ 8,500,000	$ 7,400,000
Markel Corporation [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment | shares		55,430		55,430		55,430	55,430
Marketable equity securities		$ 87,300,000		$ 87,300,000			$ 78,700,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities		$ 454,800,000		$ 454,800,000
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		482,945		482,945		482,945
N2K Networks [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage		49.90%		49.90%		49.90%
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		42.20%		42.20%		42.20%
Intersection Holdings [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		18.00%		18.00%		18.00%
Payments for Advance to Affiliate				$ 30,000,000.0
Loan Receivable, Payment Terms				5 years
Loan Receivable Fixed Interest Rate				9.00%
Advances to Affiliate		$ 27,700,000		$ 27,700,000
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 (AHN) Healthcare at Home [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		40.00%		40.00%		40.00%
York Joint Venture [Member] | KIHL [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		45.00%		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