Fair Value (Fair Value Measurement Levels For All Assets, Recorded At Fair And Market Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	$ 11,897,480	$ 10,643,565
Equity securities, at fair value	1,488,355	1,288,767
Other invested assets, at fair value	1,933,277	1,796,479
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Equity securities, at fair value	1,451,271	1,222,158
Other invested assets, at fair value
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	10,165,144	9,383,989
Equity securities, at fair value	37,084	66,609
Other invested assets, at fair value
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	1,732,336	1,259,576
Equity securities, at fair value
Other invested assets, at fair value	1,933,277	1,796,479
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	624,050	681,989
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	624,050	681,989
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Obligation of U.S. States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	607,375	577,046
Obligation of U.S. States and Political Subdivisions [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Obligation of U.S. States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	607,375	577,046
Obligation of U.S. States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	3,751,270	3,449,912
Corporate Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Corporate Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	3,027,877	2,819,068
Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	723,393	630,844
Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	3,182,647	2,474,170
Asset-backed Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Asset-backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	2,178,578	1,851,137
Asset-backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	1,004,069	623,033
Commercial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	598,383	550,080
Commercial [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Commercial [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	598,383	550,080
Commercial [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Agency Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	1,015,112	965,100
Agency Residential [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Agency Residential [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	1,015,112	965,100
Agency Residential [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Non-agency Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	4,976	3,164
Non-agency Residential [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Non-agency Residential [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	4,976	3,164
Non-agency Residential [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Foreign Government Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	713,349	742,238
Foreign Government Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Foreign Government Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	713,349	742,238
Foreign Government Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Foreign Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	1,400,318	1,199,866
Foreign Corporate Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value
Foreign Corporate Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	1,395,444	1,194,167
Foreign Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, market value	$ 4,874	$ 5,699