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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 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 $ 12,745,692 $ 11,856,835
Equity securities 8,688,860 7,671,912
Short-term investments, available-for-sale 2,039,061 2,669,262
Total Investments 23,473,613 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 3,128,852 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 938,735 765,538
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 3,876,335 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 1,543,626 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 2,073,556 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 479,922 526,793
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 1,204 1,640
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 703,462 668,496
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 12,745,692 11,856,835
Equity securities 8,688,860 7,671,912
Short-term investments, available-for-sale 2,039,061 2,669,262
Total Investments 23,473,613 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 8,687,924 7,671,013
Short-term investments, available-for-sale 1,872,751 2,510,164
Total Investments 10,560,675 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 12,745,692 11,856,835
Equity securities 0 0
Short-term investments, available-for-sale 166,310 159,098
Total Investments 12,912,002 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 936 899
Short-term investments, available-for-sale 0 0
Total Investments 936 899
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 3,128,852 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 3,128,852 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 938,735 765,538
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 938,735 765,538
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 3,876,335 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 3,876,335 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 1,543,626 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 1,543,626 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 2,073,556 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 2,073,556 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 479,922 526,793
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 479,922 526,793
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] | 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 1,204 1,640
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 1,204 1,640
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 703,462 668,496
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 703,462 668,496
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 3,224,329 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 3,223,393 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 936 899
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 5,464,531 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 5,464,531 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