Investments (Narrative) (Details) £ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Apr. 30, 2023 USD ($)	Jan. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2021 USD ($) investment	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) investment	Dec. 31, 2023 GBP (£) shares
Schedule of Investments [Line Items]
Money market investments					$ 5,600,000	$ 7,700,000
Marketable equity securities					690,153,000	609,921,000
Investments in Marketable Securities					4,600,000	42,100,000
Payments to acquire marketable securities					6,162,000	40,518,000	$ 48,036,000
Portion of 2022 investments in marketable equity securities that settled in January 2023		$ 1,500,000
Cumulative realized gain on marketable securities					13,000,000	58,100,000	46,000,000
Proceeds from sales of marketable equity securities					61,979,000	102,040,000	65,499,000
Payments for Advance to Affiliate					30,000,000	0	0
Gain on sale of investment in affiliate					15,000	604,000	0
Cumulative undistributed income in investments in affiliates					36,900,000	49,100,000
Equity Method Investment Impairment				$ 6,600,000			$ 6,600,000
Number of Equity Method Investments Impaired | investment				1			1
Investments in Affiliates					186,480,000	186,419,000
Cost method investments					74,000,000	66,700,000
Net gain on cost method investments					3,104,000	6,883,000	$ 11,756,000
Impairment of a cost method investment					$ 500,000	$ 1,305,000	0
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					55,430	55,430		55,430
Marketable equity securities					$ 78,700,000	$ 73,000,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities					$ 401,200,000
Berkshire Hathaway [Member] | Class A Common Stock [Member]
Schedule of Investments [Line Items]
Number of shares held in investment | shares					422			422
Berkshire Hathaway [Member] | Class B Common Stock [Member]
Schedule of Investments [Line Items]
Number of shares held in investment | shares					482,945			482,945
Realm [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage					42.20%			42.20%
N2K Networks [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage					49.90%			49.90%
Intersection [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 Fixed Interest Rate	9.00%
Loan Receivable, Payment Term	5 years
Advances to Affiliate					$ 28,800,000
Private Equity Fund Investment [Member]
Schedule of Investments [Line Items]
Investments in Affiliates					$ 56,600,000	68,900,000
Private Equity Fund Investment [Member] | Minimum [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage					4.00%			4.00%
Private Equity Fund Investment [Member] | Maximum [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage					10.00%			10.00%
Graham Healthcare Group [Member] | Related Party [Member]
Schedule of Investments [Line Items]
Operating Revenues					$ 15,600,000	$ 13,900,000	$ 10,900,000
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					40.00%			40.00%
Gain on sale of investment in affiliate			$ 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%
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%
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%
Loan Receivable Fixed Interest Rate					7.00%
Loan Receivable, Payment Term					25 years
Advances to Affiliate | £								£ 19.9
Cumulative Payments For Advance to Affiliate | £								£ 22.0