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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value $ 10,060,472 $ 9,970,909
Equity securities 5,664,247 7,590,755
Short-term investments, available-for-sale 929,272 1,196,248
Total Investments 16,653,991 18,757,912
U.S. Treasury securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 354,895 284,786
U.S. government-sponsored enterprises [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 374,808 342,580
Obligations of states, municipalities and political subdivisions [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 4,205,455 4,189,882
Foreign governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 1,515,396 1,542,004
Commercial mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 1,878,978 1,817,845
Residential mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 907,330 888,073
Asset-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 8,281 11,048
Corporate bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 815,329 894,691
Fair value, recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 10,060,472 9,970,909
Equity securities 5,664,247 7,590,755
Short-term investments, available-for-sale 929,272 1,196,248
Total Investments 16,653,991 18,757,912
Fair value, recurring [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Equity securities 5,548,599 7,544,763
Short-term investments, available-for-sale 823,932 1,093,799
Total Investments 6,372,531 8,638,562
Fair value, recurring [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 10,060,472 9,970,909
Equity securities 0 0
Short-term investments, available-for-sale 105,340 102,449
Total Investments 10,165,812 10,073,358
Fair value, recurring [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Equity securities 115,648 45,992
Short-term investments, available-for-sale 0 0
Total Investments 115,648 45,992
Fair value, recurring [Member] | U.S. Treasury securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 354,895 284,786
Fair value, recurring [Member] | U.S. Treasury securities [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | U.S. Treasury securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 354,895 284,786
Fair value, recurring [Member] | U.S. Treasury securities [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 374,808 342,580
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 374,808 342,580
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 4,205,455 4,189,882
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 4,205,455 4,189,882
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Foreign governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 1,515,396 1,542,004
Fair value, recurring [Member] | Foreign governments [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Foreign governments [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 1,515,396 1,542,004
Fair value, recurring [Member] | Foreign governments [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 1,878,978 1,817,845
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 1,878,978 1,817,845
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Residential mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 907,330 888,073
Fair value, recurring [Member] | Residential mortgage-backed securities [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Residential mortgage-backed securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 907,330 888,073
Fair value, recurring [Member] | Residential mortgage-backed securities [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Asset-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 8,281 11,048
Fair value, recurring [Member] | Asset-backed securities [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Asset-backed securities [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 8,281 11,048
Fair value, recurring [Member] | Asset-backed securities [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Corporate bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 815,329 894,691
Fair value, recurring [Member] | Corporate bonds [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Corporate bonds [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 815,329 894,691
Fair value, recurring [Member] | Corporate bonds [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value 0 0
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 2,130,162 2,509,182
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 2,014,514 2,463,190
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 115,648 45,992
Fair value, recurring [Member] | Industrial, consumer and all other [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 3,534,085 5,081,573
Fair value, recurring [Member] | Industrial, consumer and all other [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 3,534,085 5,081,573
Fair value, recurring [Member] | Industrial, consumer and all other [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Fair value, recurring [Member] | Industrial, consumer and all other [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities $ 0 $ 0