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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity investments, estimated fair value $ 14,372,732 $ 11,856,835
Equity securities 9,577,871 7,671,912
Short-term investments 2,571,382 2,669,262
Investments 26,521,985 22,198,009
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 3,784,713 2,913,959
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 1,142,814 765,538
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 4,029,305 3,733,183
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 1,783,421 1,306,778
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 2,243,658 1,940,448
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 470,422 526,793
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 918,399 670,136
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 14,372,732 11,856,835
Equity securities 9,577,871 7,671,912
Short-term investments 2,571,382 2,669,262
Investments 26,521,985 22,198,009
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 9,576,877 7,671,013
Short-term investments 2,402,099 2,510,164
Investments 11,978,976 10,181,177
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 14,372,732 11,856,835
Equity securities 0 0
Short-term investments 169,283 159,098
Investments 14,542,015 12,015,933
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 994 899
Short-term investments 0 0
Investments 994 899
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 3,784,713 2,913,959
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 3,784,713 2,913,959
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 1,142,814 765,538
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 1,142,814 765,538
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 4,029,305 3,733,183
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 4,029,305 3,733,183
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 1,783,421 1,306,778
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 1,783,421 1,306,778
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 2,243,658 1,940,448
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 2,243,658 1,940,448
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 470,422 526,793
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 470,422 526,793
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] | 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 918,399 670,136
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 918,399 670,136
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 3,695,369 2,953,588
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 3,694,375 2,952,689
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 994 899
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 5,882,502 4,718,324
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 5,882,502 4,718,324
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