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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
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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Fair Value Measurement [Line Items]

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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.

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Aggregation of the liabilities reported on the balance sheet measured at fair value on a recurring basis by the entity.

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