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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
Jun. 30, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: $ 3,157,398 $ 2,985,161
Equity securities: 645,407 529,631
Short-term investments: 362,197 253,299
Equity contracts [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 405 3
Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 3,157,398 2,985,161
Equity securities: 645,407 529,631
Short-term investments: 362,197 253,299
Note receivable 5,635 5,557
Assets, fair value disclosure 4,170,637 3,773,648
Liabilities, fair value 2,127 4,854
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 23,895 25,003
Equity securities: 543,735 430,973
Short-term investments: 322,570 236,515
Note receivable 0 0
Assets, fair value disclosure 890,200 692,491
Liabilities, fair value 405 3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 3,133,503 2,960,158
Equity securities: 33,778 31,433
Short-term investments: 39,627 16,784
Note receivable 5,635 5,557
Assets, fair value disclosure 3,212,543 3,013,932
Liabilities, fair value 1,722 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: 1,206 1,445
Short-term investments: 0 0
Note receivable 0 0
Assets, fair value disclosure 1,206 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: 23,895 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: 23,895 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: 2,713,171 2,620,132
Fair Value, Measurements, Recurring [Member] | Municipal 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] | Municipal Securities [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities: 2,713,171 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: 22,916 30,952
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: 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: 22,916 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: 181,943 105,524
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: 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: 181,943 105,524
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: 172,466 165,789
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: 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: 172,466 165,789
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: 43,007 37,761
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: 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: 43,007 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: 543,735 430,973
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities: 543,735 430,973
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: 33,778 31,433
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 2 [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: 33,778 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: 1,206 1,445
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 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: 1,206 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: 66,688 65,780
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term investments: 72,428 48,256
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: 32,801 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: 39,627 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: 289,769 205,043
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: 289,769 205,043
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 1,722 4,851
Liabilities, fair value 1,722 4,851
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 2 [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 1,722 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 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liability 405 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