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Investments (Narrative) (Details) £ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2023 USD ($) shares | Mar. 31, 2022 USD ($) | Mar. 31, 2023 GBP (£) shares | Dec. 31, 2022 USD ($) shares
Schedule of Investments [Line Items] |  |  |  |
Money Market Investments | $ 2,200,000 |  |  | $ 7,700,000
Marketable equity securities | 603,543,000 |  |  | 609,921,000
Investments in Marketable Securities | 4,600,000 |  |  |
Payments to Acquire Marketable Securities | 6,162,000 | $ 0 |  |
Cumulative realized (loss) gain on marketable securities | (12,200,000) | 41,600,000 |  |
Proceeds from sales of marketable equity securities | 29,028,000 | 55,731,000 |  |
Gain on sale of investments in affiliates | 15,000 | 604,000 |  |
Cumulative undistributed income in investments in affiliates | 50,100,000 |  |  | 49,100,000
Cost method investments | 69,400,000 |  |  | $ 66,700,000
Gain on a cost method investment | $ 1,831,000 | 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
Marketable equity securities | $ 70,800,000 |  |  | $ 73,000,000
Berkshire Hathaway [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Marketable equity securities | $ 345,600,000 |  |  |
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 | 482,945 |  | 482,945 |
N2K Networks [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity Method Investment, Diluted Ownership Percentage | 49.90% |  | 49.90% |
Intersection Holdings [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity Method Investment, Ownership Percentage | 18.00% |  | 18.00% |
Graham Healthcare Group [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Revenue from Related Parties | $ 3,500,000 | 3,300,000 |  |
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 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% |
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% |
Cumulative payments for advance to affiliate | £ |  |  | £ 22.0 |
Loan Receivable, Payment Terms | 25 years |  |  |
Loan Receivable Fixed Interest Rate | 7.00% |  |  |
Advances to Affiliate | £ |  |  | £ 20.4 |

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