Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value		$ 10,043,188	$ 9,940,670
Equity securities		5,720,945	0
Short-term investments		1,077,696	2,160,974
Total investments		11,120,884	18,069,491
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		10,043,188	9,940,670
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		246,642	160,613
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		359,322	363,520
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		4,352,930	4,566,562
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		1,559,604	1,489,228
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		1,650,199	1,234,326
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		880,172	856,168
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		19,408	34,728
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		974,911	1,235,525
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	[1]		5,967,847
Insurance, Banks And Other Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	[1]		2,103,033
Industrial, Consumer And All Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	[1]		3,864,814
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value		10,043,188	9,940,670
Equity securities		5,720,945	5,967,847	[2]
Short-term investments		1,077,696	2,160,974
Total investments		16,841,829	18,069,491	[2]
Fair Value, Measurements, 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		5,667,217	5,799,038	[2]
Short-term investments		981,616	2,065,749
Total investments		6,648,833	7,864,787	[2]
Fair Value, Measurements, 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		10,043,188	9,940,670
Equity securities		0	0	[2]
Short-term investments		96,080	95,225
Total investments		10,139,268	10,035,895	[2]
Fair Value, Measurements, 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		53,728	168,809	[2]
Short-term investments		0	0
Total investments		53,728	168,809	[2]
Fair Value, Measurements, 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		246,642	160,613
Fair Value, Measurements, 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, Measurements, 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		246,642	160,613
Fair Value, Measurements, 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, Measurements, 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		359,322	363,520
Fair Value, Measurements, 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, Measurements, 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		359,322	363,520
Fair Value, Measurements, 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, Measurements, 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		4,352,930	4,566,562
Fair Value, Measurements, 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, Measurements, 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		4,352,930	4,566,562
Fair Value, Measurements, 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, Measurements, 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		1,559,604	1,489,228
Fair Value, Measurements, 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, Measurements, 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		1,559,604	1,489,228
Fair Value, Measurements, 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, Measurements, 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		1,650,199	1,234,326
Fair Value, Measurements, 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, Measurements, 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		1,650,199	1,234,326
Fair Value, Measurements, 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, Measurements, 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		880,172	856,168
Fair Value, Measurements, 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, Measurements, 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		880,172	856,168
Fair Value, Measurements, 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, Measurements, 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		19,408	34,728
Fair Value, Measurements, 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, Measurements, 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		19,408	34,728
Fair Value, Measurements, 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, Measurements, 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		974,911	1,235,525
Fair Value, Measurements, 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, Measurements, 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		974,911	1,235,525
Fair Value, Measurements, 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, Measurements, Recurring [Member] | Insurance, Banks And Other Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		1,930,539	2,103,033	[2]
Fair Value, Measurements, 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		1,876,811	1,934,224	[2]
Fair Value, Measurements, 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	[2]
Fair Value, Measurements, 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		53,728	168,809	[2]
Fair Value, Measurements, Recurring [Member] | Industrial, Consumer And All Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		3,790,406	3,864,814	[2]
Fair Value, Measurements, 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		3,790,406	3,864,814	[2]
Fair Value, Measurements, 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	[2]
Fair Value, Measurements, 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	[2]

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01 and equity securities are no longer classified as available-for-sale. Prior periods have not been restated to conform to the current presentation. See note 1.

[2] Effective January 1, 2018, the Company adopted ASU No. 2016-01 and equity securities are no longer classified as available-for-sale. Prior periods have not been restated to conform to the current presentation. See note 1.