Investments (Narrative) (Details) £ in Millions	1 Months Ended	3 Months Ended
	May 31, 2024 USD ($)	Mar. 31, 2026 USD ($) shares	Mar. 31, 2025 USD ($)	Mar. 31, 2026 GBP (£) shares	Dec. 31, 2025 USD ($) shares
Schedule of Investments [Line Items]
Money Market Investments		$ 7,500,000			$ 5,300,000
Marketable equity securities		966,719,000			1,081,938,000
Payments to Acquire Marketable Securities		3,698,000	$ 4,823,000
Cumulative realized net gains on sales of marketable securities		45,500,000
Proceeds from sales of marketable equity securities		49,994,000	0
Cumulative undistributed income in investments in affiliates		46,900,000			30,000,000.0
Cost method investments		$ 49,400,000			$ 49,400,000
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		$ 4,100,000	$ 3,800,000
Markel Corporation [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment | shares		55,430		55,430	55,430
Marketable equity securities		$ 106,100,000			$ 119,200,000
Berkshire Hathaway [Member]
Schedule of Investments [Line Items]
Marketable equity securities		$ 534,000,000.0
Berkshire Hathaway [Member] | Common Class A [Member]
Schedule of Investments [Line Items]
Number of Shares Held in Investment | shares		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
Residential Home Health Illinois [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		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%
Mary Free Bed at Home [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		40.00%		40.00%
Allegheny Health Network Healthcare at Home [Member] | Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		40.00%		40.00%
N2K Networks [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage		50.40%		50.40%
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		41.40%		41.40%
Intersection [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Diluted Ownership Percentage		25.20%		25.20%
York Joint Venture [Member] | KIHL [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage		45.00%		45.00%
Loan Receivable Fixed Interest Rate		7.00%
Cumulative payments for advance to affiliate | £				£ 22.0
Loan Receivable, Payment Terms		25 years
Advances to Affiliate | £				£ 18.8