v3.26.1
Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Thousands
Mar. 31, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value $ 17,943,781 $ 17,797,986
Equity securities 12,302,672 13,004,312
Short-term investments, available-for-sale 2,026,147 2,033,662
Total Investments 32,272,600 32,835,960
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 17,943,781 17,797,986
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 6,001,802 5,927,907
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 1,664,622 1,640,234
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 3,672,453 3,672,468
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 3,424,257 3,374,915
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 2,926,764 2,901,120
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 54,055 65,758
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 199,828 215,584
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 17,943,781 17,797,986
Equity securities 12,302,672 13,004,312
Short-term investments, available-for-sale 2,026,147 2,033,662
Total Investments 32,272,600 32,835,960
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 12,302,672 13,004,312
Short-term investments, available-for-sale 1,824,361 1,843,163
Total Investments 14,127,033 14,847,475
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 17,943,781 17,797,986
Equity securities 0 0
Short-term investments, available-for-sale 201,786 190,499
Total Investments 18,145,567 17,988,485
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 0
Short-term investments, available-for-sale 0 0
Total Investments 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 securities, estimated fair value 6,001,802 5,927,907
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 6,001,802 5,927,907
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 1,664,622 1,640,234
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 1,664,622 1,640,234
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 3,672,453 3,672,468
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 3,672,453 3,672,468
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 3,424,257 3,374,915
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 3,424,257 3,374,915
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 2,926,764 2,901,120
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 2,926,764 2,901,120
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 54,055 65,758
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 54,055 65,758
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 199,828 215,584
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 199,828 215,584
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 4,884,952 5,441,617
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 4,884,952 5,441,617
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 0
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 7,417,720 7,562,695
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 7,417,720 7,562,695
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