Fair Value Of Financial Instruments (Schedule Of Fair Value Of Financial Instruments) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 797,885	$ 797,700
U.S. Government And Government Agencies [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	35,298	91,953
Trading securities	1,067,673	1,229,720
U.S. Government And Government Agencies [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	35,298	91,953
Trading securities	1,067,673	1,229,720
U.S. Government And Government Agencies [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	35,298	65,234
Trading securities	858,948	1,088,967
U.S. Government And Government Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities		26,719
Trading securities	208,725	140,753
U.S. Government And Government Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Non-U.S. Government And Government Agencies [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities		145,384
Trading securities	225,782	120,793
Non-U.S. Government And Government Agencies [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities		145,384
Trading securities	225,782	120,793
Non-U.S. Government And Government Agencies [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Non-U.S. Government And Government Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities		145,384
Trading securities	225,782	120,793
Non-U.S. Government And Government Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
States, Municipalities And Political Subdivisions [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	33,107	118,177
Trading securities	103,180	127,436
States, Municipalities And Political Subdivisions [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	33,107	118,177
Trading securities	103,180	127,436
States, Municipalities 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]
Available for sale securities
Trading securities
States, Municipalities And Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	33,107	118,177
Trading securities	103,180	127,436
States, Municipalities And Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Corporate Debt [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	216,561	536,335
Trading securities	2,234,717	1,990,215
Corporate Debt [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	216,561	536,335
Trading securities	2,234,717	1,990,215
Corporate Debt [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Corporate Debt [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	216,561	536,335
Trading securities	2,234,717	1,990,215
Corporate Debt [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Mortgage-Backed [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities
Mortgage-Backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	1,806,200	1,579,324
Mortgage-Backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	247,463	172,559
Carrying Amount [Member] | Mortgage-Backed [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	2,053,663	1,751,883
Fair Value [Member] | Mortgage-Backed [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	2,053,663	1,751,883
Asset-Backed [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities
Asset-Backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	498,993	500,343
Asset-Backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	95,607	48,707
Carrying Amount [Member] | Asset-Backed [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	594,600	549,050
Fair Value [Member] | Asset-Backed [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	594,600	549,050
Equity Securities [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	424,104	174,976
Equity Securities [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	424,104	174,976
Equity Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	424,104	174,976
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities
Other Invested Assets [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets	552,074	347,632
Other Invested Assets [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets	552,074	347,632
Other Invested Assets [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets
Other Invested Assets [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets
Other Invested Assets [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets	552,074	347,632
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	284,966	891,849
Trading securities	6,279,615	5,769,097
Total fixed maturity investments	6,564,581	6,660,946
Total investments	7,540,759	7,183,554
Senior notes	797,885	797,700
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	284,966	891,849
Trading securities	6,279,615	5,769,097
Total fixed maturity investments	6,564,581	6,660,946
Total investments	7,540,759	7,183,554
Senior notes	865,868	850,570
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Total fixed maturity investments
Total investments	1,318,350	1,329,177
Senior notes
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Total fixed maturity investments
Total investments	5,327,265	5,285,479
Senior notes	865,868	850,570
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities
Trading securities
Total fixed maturity investments
Total investments	895,144	568,898
Senior notes