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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | $ 15,745,539 | $ 14,372,732
Equity securities | 11,784,521 | 9,577,871
Short-term investments | 2,524,910 | 2,571,382
Investments | 30,054,970 | 26,521,985
Fixed maturities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | 15,745,539 | 14,372,732
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 | 5,087,858 | 3,784,713
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 | 1,346,236 | 1,142,814
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 | 3,619,492 | 4,029,305
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 | 2,706,523 | 2,447,218
Agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | 2,649,813 | 2,381,010
Non-agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | 118,030 | 333,070
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 | 217,587 | 254,602
Fair Value, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 11,784,521 | 9,577,871
Short-term investments | 2,524,910 | 2,571,382
Investments | 30,054,970 | 26,521,985
Fair Value, Recurring [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 11,783,137 | 9,576,877
Short-term investments | 2,363,736 | 2,402,099
Investments | 14,146,873 | 11,978,976
Fair Value, Recurring [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Short-term investments | 161,174 | 169,283
Investments | 15,906,713 | 14,542,015
Fair Value, Recurring [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 1,384 | 994
Short-term investments | 0 | 0
Investments | 1,384 | 994
Fair Value, Recurring [Member] | Fixed maturities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | 15,745,539 | 14,372,732
Fair Value, Recurring [Member] | Fixed maturities [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] | Fixed maturities [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 | 15,745,539 | 14,372,732
Fair Value, Recurring [Member] | Fixed maturities [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. 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 | 5,087,858 | 3,784,713
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 | 5,087,858 | 3,784,713
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 | 1,346,236 | 1,142,814
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 | 1,346,236 | 1,142,814
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 | 3,619,492 | 4,029,305
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 | 3,619,492 | 4,029,305
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 | 2,706,523 | 2,447,218
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 | 2,706,523 | 2,447,218
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] | Agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | 2,649,813 | 2,381,010
Fair Value, Recurring [Member] | Agency mortgage-backed securities | 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] | Agency mortgage-backed securities | 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 | 2,649,813 | 2,381,010
Fair Value, Recurring [Member] | Agency mortgage-backed securities | 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] | Non-agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity investments, estimated fair value | 118,030 | 333,070
Fair Value, Recurring [Member] | Non-agency mortgage-backed securities | 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] | Non-agency mortgage-backed securities | 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 | 118,030 | 333,070
Fair Value, Recurring [Member] | Non-agency mortgage-backed securities | 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 | 217,587 | 254,602
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 | 217,587 | 254,602
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 | 4,970,120 | 3,695,369
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 | 4,968,736 | 3,694,375
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 | 1,384 | 994
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 | 6,814,401 | 5,882,502
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 | 6,814,401 | 5,882,502
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

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