Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 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	$ 11,642,212	$ 11,120,884
Equity securities	6,875,933	5,720,945
Total Investments	18,518,145	16,841,829
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	10,258,640	10,043,188
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	297,673	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	356,859	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	4,362,197	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	1,632,575	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	1,758,022	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	909,969	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	11,196	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	930,149	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	10,258,640	10,043,188
Equity securities	6,875,933	5,720,945
Short-term investments	1,383,572	1,077,696
Total Investments	18,518,145	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	6,837,945	5,667,217
Short-term investments	1,279,382	981,616
Total Investments	8,117,327	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	10,258,640	10,043,188
Equity securities	0	0
Short-term investments	104,190	96,080
Total Investments	10,362,830	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	37,988	53,728
Short-term investments	0	0
Total Investments	37,988	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	297,673	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	297,673	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	356,859	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	356,859	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	4,362,197	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	4,362,197	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	1,632,575	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	1,632,575	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	1,758,022	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	1,758,022	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	909,969	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	909,969	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	11,196	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	11,196	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	930,149	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	930,149	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	2,257,755	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	2,219,767	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	37,988	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	4,618,178	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	4,618,178	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