Investments (Narrative) (Details) £ in Millions	1 Months Ended	3 Months Ended			6 Months Ended
	Apr. 30, 2023 USD ($)	Jun. 30, 2023 USD ($) shares	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Jun. 30, 2023 USD ($) shares	Jun. 30, 2022 USD ($)	Jun. 30, 2023 GBP (£) shares	Dec. 31, 2022 USD ($) shares
Schedule of Investments [Line Items]
Money Market Investments		$ 6,000,000			$ 6,000,000			$ 7,700,000
Marketable equity securities		648,756,000			648,756,000			609,921,000
Investments in Marketable Securities					4,600,000
Payments to Acquire Marketable Securities					6,162,000	$ 31,468,000
Cumulative realized gain on marketable securities					13,000,000	39,100,000
Proceeds from sales of marketable equity securities					61,979,000	74,233,000
Payments for Advance to Affiliate					30,000,000	0
Gain on sale of investments in affiliates		0	$ 0		15,000	604,000
Cumulative undistributed income in investments in affiliates		42,600,000			42,600,000			49,100,000
Cost method investments		73,400,000			73,400,000			$ 66,700,000
Gain on cost method investments		$ 1,273,000	0		$ 3,104,000	0
Equity Securities [Member] | 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%
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		$ 76,700,000			$ 76,700,000			$ 73,000,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities		$ 383,200,000			$ 383,200,000
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
Realm [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage		42.20%			42.20%		42.20%
N2K Networks [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage		49.90%			49.90%		49.90%
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
Loan Receivable Fixed Interest Rate	9.00%
Loan Receivable, Payment Terms	5 years
Advances to Affiliate		$ 29,700,000			$ 29,700,000
Graham Healthcare Group [Member] | Related Party [Member]
Schedule of Investments [Line Items]
Operating Revenues		$ 3,900,000	$ 3,700,000		$ 7,400,000	$ 7,000,000
Graham Healthcare Group [Member] | Residential Home Health Illinois [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		40.00%			40.00%		40.00%
Graham Healthcare Group [Member] | Residential Hospice Illinois [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		40.00%			40.00%		40.00%
Gain on sale of investments in affiliates				$ 600,000
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%		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%		40.00%
Graham Healthcare Group [Member] | Residential Home Health Illinois and Residential Hospice Illinois [Member]
Schedule of Investments [Line Items]
Additional investment in equity affiliates				$ 18,500,000
KIHL [Member] | York Joint Venture [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		45.00%			45.00%		45.00%
Loan Receivable Fixed Interest Rate					7.00%
Loan Receivable, Payment Terms					25 years
Advances to Affiliate | £							£ 20.4
Cumulative payments for advance to affiliate | £							£ 22.0