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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, 2018
Dec. 31, 2017
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) $ 2,985,161,000 $ 2,892,777,000
Equity securities (cost $544,082; $474,197) 529,631,000 537,240,000
Short-term investments (cost $254,518; $302,693) 253,299,000 302,711,000
Total Return Swap [Member]    
Fair Value Measurement [Line Items]    
Liabilities fair value 4,851,000 1,200,000
Equity Contract [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 3,000 123,000
Liabilities fair value   123,000
Fair Value, Measurements, Recurring [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 2,985,161,000 2,892,777,000
Equity securities (cost $544,082; $474,197) 529,631,000 537,240,000
Short-term investments (cost $254,518; $302,693) 253,299,000 302,711,000
Other assets: 5,557,000 5,565,000
Total assets at fair value 3,773,648,000 3,738,293,000
Liabilities fair value 4,854,000 1,323,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 25,003,000 13,236,000
Equity securities (cost $544,082; $474,197) 430,973,000 429,367,000
Short-term investments (cost $254,518; $302,693) 236,515,000 300,691,000
Other assets: 0 0
Total assets at fair value 692,491,000 743,294,000
Liabilities fair value 3,000 123,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 2,960,158,000 2,879,541,000
Equity securities (cost $544,082; $474,197) 31,433,000 34,869,000
Short-term investments (cost $254,518; $302,693) 16,784,000 2,020,000
Other assets: 5,557,000 5,565,000
Total assets at fair value 3,013,932,000 2,921,995,000
Liabilities fair value 4,851,000 1,200,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Equity securities (cost $544,082; $474,197) 1,445,000 1,481,000
Short-term investments (cost $254,518; $302,693) 0 0
Other assets: 0 0
Total assets at fair value 1,445,000 1,481,000
Liabilities fair value 0 0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 25,003,000 13,236,000
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 25,003,000 13,236,000
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 2,620,132,000 2,556,532,000
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 2,620,132,000 2,556,532,000
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 30,952,000 27,165,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 30,952,000 27,165,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 105,524,000 137,542,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 105,524,000 137,542,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 165,789,000 105,202,000
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 165,789,000 105,202,000
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 37,761,000 53,100,000
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 37,761,000 53,100,000
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Fixed maturity securities (amortized cost $2,969,541; $2,823,230) 0 0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 430,973,000 429,367,000
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 430,973,000 429,367,000
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 0 0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 0 0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 31,433,000 34,869,000
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 0 0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 31,433,000 34,869,000
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 0 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 1,445,000 1,481,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 0 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 0 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 1,445,000 1,481,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds Net Asset Value [Member]    
Fair Value Measurement [Line Items]    
Equity securities (cost $544,082; $474,197) 65,780,000 71,523,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 48,256,000 32,018,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 31,472,000 29,998,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 16,784,000 2,020,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 0 0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 205,043,000 270,693,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 205,043,000 270,693,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 0 0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Short-term investments (cost $254,518; $302,693) 0 0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 4,851,000 1,200,000
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 0 0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 4,851,000 1,200,000
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 0 0
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 3,000 123,000
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly 0 0
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Derivative liabilitly $ 0 $ 0