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Fair Value Measurements (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) - USD ($) | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | $ 3,738,293,000 | $ 3,548,227,000
Fair value of liabilities measured on a recurring basis | 1,323,000 | 785,000
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 743,294,000 | 684,172,000
Fair value of liabilities measured on a recurring basis | 123,000 | 20,000
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,921,995,000 | 2,854,987,000
Fair value of liabilities measured on a recurring basis | 1,200,000 | 765,000
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 1,481,000 | 9,068,000
Fair value of liabilities measured on a recurring basis | 0 | 0
Financing Receivable [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 5,565,000 |
Financing Receivable [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 |
Financing Receivable [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 5,565,000 |
Financing Receivable [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 |
Money Market Funds [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 302,711,000 | 375,680,000
Money Market Funds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 300,691,000 | 355,447,000
Money Market Funds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,020,000 | 20,233,000
Money Market Funds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,892,777,000 | 2,814,553,000
Fixed Maturity Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 13,236,000 | 12,275,000
Fixed Maturity Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,879,541,000 | 2,802,278,000
Fixed Maturity Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | US Treasury and Government [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 13,236,000 | 12,275,000
Fixed Maturity Securities [Member] | US Treasury and Government [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 13,236,000 | 12,275,000
Fixed Maturity Securities [Member] | US Treasury and Government [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | US Treasury and Government [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,556,532,000 | 2,449,292,000
Fixed Maturity Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,556,532,000 | 2,449,292,000
Fixed Maturity Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 27,165,000 | 39,777,000
Fixed Maturity Securities [Member] | Collateralized Mortgage Backed Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Collateralized Mortgage Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 27,165,000 | 39,777,000
Fixed Maturity Securities [Member] | Collateralized Mortgage Backed Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Corporate Debt Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 137,542,000 | 189,688,000
Fixed Maturity Securities [Member] | Corporate Debt Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Corporate Debt Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 137,542,000 | 189,688,000
Fixed Maturity Securities [Member] | Corporate Debt Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Collateralized Debt Obligations [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 105,202,000 | 86,525,000
Fixed Maturity Securities [Member] | Collateralized Debt Obligations [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Collateralized Debt Obligations [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 105,202,000 | 86,525,000
Fixed Maturity Securities [Member] | Collateralized Debt Obligations [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Collateralized Auto Loans [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 53,100,000 | 36,996,000
Fixed Maturity Securities [Member] | Collateralized Auto Loans [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fixed Maturity Securities [Member] | Collateralized Auto Loans [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 53,100,000 | 36,996,000
Fixed Maturity Securities [Member] | Collateralized Auto Loans [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Equity Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 537,240,000 | 357,327,000
Equity Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 429,367,000 | 316,450,000
Equity Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 34,869,000 | 31,809,000
Equity Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 1,481,000 | 9,068,000
Equity Securities [Member] | Common Stock [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 429,367,000 | 316,450,000
Equity Securities [Member] | Common Stock [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 429,367,000 | 316,450,000
Equity Securities [Member] | Common Stock [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis |  | 0
Equity Securities [Member] | Common Stock [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 34,869,000 | 31,809,000
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 34,869,000 | 31,809,000
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Equity Securities [Member] | Private Equity Funds [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 1,481,000 | 9,068,000
Equity Securities [Member] | Private Equity Funds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Equity Securities [Member] | Private Equity Funds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Equity Securities [Member] | Private Equity Funds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 1,481,000 | 9,068,000
Short-term Investments [Member] | Short-term Debt [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 32,018,000 | 90,626,000
Short-term Investments [Member] | Short-term Debt [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 29,998,000 | 70,393,000
Short-term Investments [Member] | Short-term Debt [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,020,000 | 20,233,000
Short-term Investments [Member] | Short-term Debt [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Money Market Funds [Member] | Money Market Funds [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 270,693,000 | 285,054,000
Money Market Funds [Member] | Money Market Funds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 270,693,000 | 285,054,000
Money Market Funds [Member] | Money Market Funds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Money Market Funds [Member] | Money Market Funds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Total Return Swap [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 667,000
Fair value of liabilities measured on a recurring basis | 1,200,000 | 765,000
Total Return Swap [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fair value of liabilities measured on a recurring basis | 0 | 0
Total Return Swap [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 667,000
Fair value of liabilities measured on a recurring basis | 1,200,000 | 765,000
Total Return Swap [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 0 | 0
Fair value of liabilities measured on a recurring basis | 0 | 0
Equity Contract [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of liabilities measured on a recurring basis | 123,000 | 20,000
Equity Contract [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of liabilities measured on a recurring basis | 123,000 | 20,000
Equity Contract [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of liabilities measured on a recurring basis | 0 | 0
Equity Contract [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of liabilities measured on a recurring basis | $ 0 | $ 0

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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
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