Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	$ 10,204,025	$ 10,043,188
Equity securities	6,978,874	5,720,945
Short-term investments, available-for-sale	1,545,393	1,077,696
Total Investments	18,728,292	16,841,829
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	296,956	246,642
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	359,973	359,322
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	4,320,108	4,352,930
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	1,615,614	1,559,604
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	1,790,083	1,650,199
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	909,669	880,172
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	9,924	19,408
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	901,698	974,911
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	10,204,025	10,043,188
Equity securities	6,978,874	5,720,945
Short-term investments, available-for-sale	1,545,393	1,077,696
Total Investments	18,728,292	16,841,829
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	6,934,385	5,667,217
Short-term investments, available-for-sale	1,444,357	981,616
Total Investments	8,378,742	6,648,833
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	10,204,025	10,043,188
Equity securities	0	0
Short-term investments, available-for-sale	101,036	96,080
Total Investments	10,305,061	10,139,268
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	44,489	53,728
Short-term investments, available-for-sale	0	0
Total Investments	44,489	53,728
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	296,956	246,642
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	296,956	246,642
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	359,973	359,322
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	359,973	359,322
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	4,320,108	4,352,930
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	4,320,108	4,352,930
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	1,615,614	1,559,604
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	1,615,614	1,559,604
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	1,790,083	1,650,199
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	1,790,083	1,650,199
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	909,669	880,172
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	909,669	880,172
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	9,924	19,408
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	9,924	19,408
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	901,698	974,911
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	901,698	974,911
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	2,292,441	1,930,539
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	2,247,952	1,876,811
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	44,489	53,728
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	4,686,433	3,790,406
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	4,686,433	3,790,406
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