Investments (Narrative) (Details) £ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2024 USD ($)	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($)	Sep. 30, 2024 GBP (£) shares	Dec. 31, 2023 USD ($) shares
Schedule of Investments [Line Items]
Money Market Investments		$ 95,500,000		$ 95,500,000			$ 5,600,000
Marketable equity securities		825,416,000		825,416,000			690,153,000
Payments to Acquire Marketable Securities				5,000,000	$ 6,162,000
Investments in Marketable Securities					4,600,000
Cumulative realized gain on marketable securities sold or donated				20,200,000	13,000,000.0
Proceeds from sales of marketable equity securities				23,524,000	61,979,000
Payments for Advance to Affiliate				2,000,000	30,000,000
Impairment on equity method investment		14,400,000		14,400,000
Cumulative undistributed income in investments in affiliates		36,400,000		36,400,000			36,900,000
Cost method investments		74,600,000		74,600,000			$ 74,000,000.0
Impairment of cost method investments		0	$ 0	744,000	0
Gain on cost method investments		191,000	0	$ 191,000	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	$ 4,100,000	$ 12,900,000	$ 11,500,000
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		$ 86,900,000		$ 86,900,000			$ 78,700,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities		$ 514,000,000.0		$ 514,000,000.0
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		50.40%		50.40%		50.40%
Payments for Advance to Affiliate	$ 2,000,000.0
Loan Receivable Fixed Interest Rate	12.00%
Impairment on equity method investment		$ 14,400,000
Realm [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage		41.40%		41.40%		41.40%
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		$ 26,700,000		$ 26,700,000
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 | £						£ 19.3
Cumulative payments for advance to affiliate | £						£ 22.0