v3.25.2
Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value $ 16,772,408 $ 15,745,539
Equity securities 12,330,104 11,784,521
Short-term investments, available-for-sale 1,894,524 2,524,910
Total Investments 30,997,036 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 16,772,408 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 5,504,200 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 1,522,998 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 3,650,162 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 3,125,295 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 2,687,261 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 70,357 118,030
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 212,135 217,587
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 16,772,408 15,745,539
Equity securities 12,330,104 11,784,521
Short-term investments, available-for-sale 1,894,524 2,524,910
Total Investments 30,997,036 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 12,330,104 11,783,137
Short-term investments, available-for-sale 1,731,758 2,363,736
Total Investments 14,061,862 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 16,772,408 15,745,539
Equity securities 0 0
Short-term investments, available-for-sale 162,766 161,174
Total Investments 16,935,174 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 5,504,200 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 5,504,200 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 1,522,998 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 1,522,998 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 3,650,162 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 3,650,162 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 3,125,295 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 3,125,295 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 2,687,261 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 2,687,261 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 70,357 118,030
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 70,357 118,030
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 212,135 217,587
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 212,135 217,587
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 5,102,397 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 5,102,397 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 7,227,707 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 7,227,707 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