The cash outflow and other consideration given associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

No definition available.

Investments (Narrative) (Details) £ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
Investments (Narrative) (Details) £ in Millions	Sep. 30, 2025 USD ($) shares	May 31, 2024 USD ($)	Apr. 30, 2023 USD ($)	Sep. 30, 2025 USD ($) shares	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) shares	Sep. 30, 2024 USD ($)	Sep. 30, 2025 GBP (£) shares	Dec. 31, 2024 USD ($) shares
Schedule of Investments [Line Items]
Money Market Investments	$ 5,600,000			$ 5,600,000		$ 5,600,000			$ 3,900,000
Marketable equity securities	998,793,000			998,793,000		998,793,000			852,434,000
Payments to Acquire Marketable Securities						29,823,000	$ 5,000,000
Proceeds from sales of marketable equity securities						0	23,524,000
Cumulative realized net gains on sales of marketable securities							19,800,000
Cumulative realized gain on marketable securities donated						400,000	400,000
Gain in equity earnings				15,780,000	$ (13,361,000)	10,466,000	(8,470,000)
Payments for Advance to Affiliate						0	2,000,000
Cumulative undistributed income in investments in affiliates	26,800,000			26,800,000		26,800,000			38,000,000.0
Cost method investments	$ 61,100,000			61,100,000		61,100,000			$ 74,800,000
Impairment of cost method investments				1,486,000	0	14,165,000	744,000
Gain on cost method investments				0	191,000	$ 0	191,000
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				$ 4,200,000	4,400,000	$ 12,100,000	$ 12,900,000
Markel Corporation [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment \| shares	55,430			55,430		55,430		55,430	55,430
Marketable equity securities	$ 105,900,000			$ 105,900,000		$ 105,900,000			$ 95,700,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities	$ 560,600,000			$ 560,600,000		$ 560,600,000
Berkshire Hathaway [Member] \| Common Class A [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment \| shares	422			422		422		422
Berkshire Hathaway [Member] \| Common Class B [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment \| shares	481,920			481,920		481,920		481,920
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%		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%		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%		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%		40.00%
Intersection Holdings [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	23.00%			23.00%		23.00%		23.00%
Additional Investment in Equity Method Investments	$ 29,300,000
Settlement of Advance to Affiliate	19,300,000
Advances to Affiliate	5,000,000.0			$ 5,000,000.0		$ 5,000,000.0
Gain in equity earnings	$ 18,600,000
Payments for Advance to Affiliate			$ 30,000,000
N2K Networks [Member]
Schedule of Investments [Line Items]
Payments for Advance to Affiliate		$ 2,000,000.0
Equity Method Investment, Diluted Ownership Percentage	50.40%			50.40%		50.40%		50.40%
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%		41.40%
York Joint Venture [Member] \| KIHL [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage	45.00%			45.00%		45.00%		45.00%
Advances to Affiliate \| £								£ 18.8
Loan Receivable Fixed Interest Rate						7.00%
Loan Receivable, Payment Terms						25 years
Cumulative payments for advance to affiliate \| £								£ 22.0