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Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: $ 3,139,439 $ 2,985,161
Equity securities: 657,135 529,631
Short-term investments: 421,574 253,299
Equity contracts [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 237 3
Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 3,139,439 2,985,161
Equity securities: 657,135 529,631
Short-term investments: 421,574 253,299
Note receivable 5,653 5,557
Assets, fair value disclosure 4,223,801 3,773,648
Liabilities, fair value 1,404 4,854
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 23,273 25,003
Equity securities: 561,190 430,973
Short-term investments: 384,094 236,515
Note receivable 0 0
Assets, fair value disclosure 968,557 692,491
Liabilities, fair value 237 3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 3,116,166 2,960,158
Equity securities: 34,422 31,433
Short-term investments: 37,480 16,784
Note receivable 5,653 5,557
Assets, fair value disclosure 3,193,721 3,013,932
Liabilities, fair value 1,167 4,851
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Equity securities: 1,204 1,445
Short-term investments: 0 0
Note receivable 0 0
Assets, fair value disclosure 1,204 1,445
Liabilities, fair value 0 0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 23,273 25,003
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 23,273 25,003
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 2,650,547 2,620,132
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 2,650,547 2,620,132
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Mortgage Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 42,815 30,952
Fair Value, Measurements, Recurring [Member] | Mortgage Backed Securities [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Mortgage Backed Securities [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 42,815 30,952
Fair Value, Measurements, Recurring [Member] | Mortgage Backed Securities [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 221,594 105,524
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 221,594 105,524
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 173,006 165,789
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 173,006 165,789
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 28,204 37,761
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 28,204 37,761
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 561,190 430,973
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 561,190 430,973
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 34,422 31,433
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 34,422 31,433
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 1,204 1,445
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 0 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 1,204 1,445
Fair Value, Measurements, Recurring [Member] | Private Equity Funds Net Asset Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 60,319 65,780
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 39,806 48,256
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 2,326 31,472
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 37,480 16,784
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 0 0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 381,768 205,043
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 381,768 205,043
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 0 0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 0 0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 1,167 4,851
Liabilities, fair value 1,167 4,851
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 0 0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 1,167 4,851
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 0 0
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member] | Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 237 3
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 0 0
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability $ 0 $ 0