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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | $ 17,259,981 | $ 15,745,539
Equity securities | 12,787,616 | 11,784,521
Short-term investments, available-for-sale | 2,115,757 | 2,524,910
Total Investments | 32,163,354 | 30,054,970
Fixed Maturity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 17,259,981 | 15,745,539
U.S. Treasury securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 5,665,523 | 5,087,858
U.S. government-sponsored enterprises [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 1,562,068 | 1,346,236
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 securities, estimated fair value | 3,653,829 | 3,619,492
Foreign governments, agencies and supranationals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 3,282,500 | 2,706,523
Agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 2,824,277 | 2,649,813
Non-agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 67,045 | 118,030
Corporate and university bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 204,739 | 217,587
Fair value, recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 17,259,981 | 15,745,539
Equity securities | 12,787,616 | 11,784,521
Short-term investments, available-for-sale | 2,115,757 | 2,524,910
Total Investments | 32,163,354 | 30,054,970
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 securities, estimated fair value | 0 | 0
Equity securities | 12,787,616 | 11,783,137
Short-term investments, available-for-sale | 1,942,518 | 2,363,736
Total Investments | 14,730,134 | 14,146,873
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 securities, estimated fair value | 17,259,981 | 15,745,539
Equity securities | 0 | 0
Short-term investments, available-for-sale | 173,239 | 161,174
Total Investments | 17,433,220 | 15,906,713
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 securities, estimated fair value | 0 | 0
Equity securities | 0 | 1,384
Short-term investments, available-for-sale | 0 | 0
Total Investments | 0 | 1,384
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 securities, estimated fair value | 5,665,523 | 5,087,858
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 securities, 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 securities, estimated fair value | 5,665,523 | 5,087,858
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 securities, 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 securities, estimated fair value | 1,562,068 | 1,346,236
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 securities, 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 securities, estimated fair value | 1,562,068 | 1,346,236
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 securities, 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 securities, estimated fair value | 3,653,829 | 3,619,492
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 securities, 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 securities, estimated fair value | 3,653,829 | 3,619,492
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 securities, estimated fair value | 0 | 0
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 3,282,500 | 2,706,523
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 0 | 0
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 3,282,500 | 2,706,523
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, 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 securities, estimated fair value | 2,824,277 | 2,649,813
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 securities, 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 securities, estimated fair value | 2,824,277 | 2,649,813
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 securities, 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 securities, estimated fair value | 67,045 | 118,030
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 securities, 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 securities, estimated fair value | 67,045 | 118,030
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 securities, estimated fair value | 0 | 0
Fair value, recurring [Member] | Corporate and university bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 204,739 | 217,587
Fair value, recurring [Member] | Corporate and university bonds [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 0 | 0
Fair value, recurring [Member] | Corporate and university bonds [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value | 204,739 | 217,587
Fair value, recurring [Member] | Corporate and university bonds [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale fixed maturity securities, 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 | 5,284,769 | 4,970,120
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 | 5,284,769 | 4,968,736
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 | 0 | 1,384
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 | 7,502,847 | 6,814,401
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 | 7,502,847 | 6,814,401
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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