Fair Value Of Financial Instruments (Schedule Of Fair Value Of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	$ 29,085		$ 244,016
Trading fixed maturity investments	7,125,860		6,254,686
Equity securities	490,418		367,483
Other invested assets	564,702		540,409
Total investments	8,210,065		7,406,594
Senior notes	798,147		797,949
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	29,085		244,016
Trading fixed maturity investments	7,125,860		6,254,686
Total fixed maturity investments	7,154,945		6,498,702
Equity securities	490,418		367,483
Other invested assets	564,702		540,409
Total investments	8,210,065		7,406,594
Senior notes	798,147		797,949
Fair Value, Measurements, Recurring [Member] | Foreign exchange contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	2,436	[1]	2,030	[1]
Derivative liabilities	3,103	[1]	8,934	[1]
Fair Value, Measurements, Recurring [Member] | Interest rate futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,561	[1]	977	[1]
Derivative liabilities	2,082	[1]	3,467	[1]
Fair Value, Measurements, Recurring [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	29,085		33,630
Trading fixed maturity investments	1,849,492		1,278,265
Fair Value, Measurements, Recurring [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	309,471		256,756
Fair Value, Measurements, Recurring [Member] | 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			33,479
Trading fixed maturity investments	41,542		133,902
Fair Value, Measurements, Recurring [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments			176,907
Trading fixed maturity investments	2,237,187		2,254,474
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,251,957		1,818,091
Fair Value, Measurements, Recurring [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	436,211		513,198
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	29,085		244,016
Trading fixed maturity investments	7,125,860		6,254,686
Total fixed maturity investments	7,154,945		6,498,702
Equity securities	490,418		367,483
Other invested assets	564,702		540,409
Total investments	8,210,065		7,406,594
Senior notes	914,816		872,731
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Foreign exchange contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	2,436	[1]	2,030	[1]
Derivative liabilities	3,103	[1]	8,934	[1]
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Interest rate futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,561	[1]	977	[1]
Derivative liabilities	2,082	[1]	3,467	[1]
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	29,085		33,630
Trading fixed maturity investments	1,849,492		1,278,265
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	309,471		256,756
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | 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			33,479
Trading fixed maturity investments	41,542		133,902
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments			176,907
Trading fixed maturity investments	2,237,187		2,254,474
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,251,957		1,818,091
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	436,211		513,198
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	490,418		367,483
Other invested assets	0		0
Total investments	1,978,007		1,455,116
Senior notes	0		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Foreign exchange contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	[1]	0	[1]
Derivative liabilities	0	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Interest rate futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	[1]	0	[1]
Derivative liabilities	0	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	29,085		33,630
Trading fixed maturity investments	1,458,504		1,054,003
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | 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			0
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments			0
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0		0
Other invested assets	0		0
Total investments	5,422,319		5,067,120
Senior notes	914,816		872,731
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Foreign exchange contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	2,436	[1]	2,030	[1]
Derivative liabilities	3,103	[1]	8,934	[1]
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Interest rate futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,561	[1]	977	[1]
Derivative liabilities	2,082	[1]	3,467	[1]
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	0		0
Trading fixed maturity investments	390,988		224,262
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	309,471		256,756
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | 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			33,479
Trading fixed maturity investments	41,542		133,902
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments			176,907
Trading fixed maturity investments	2,237,187		2,254,474
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,082,687		1,568,887
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	360,444		418,453
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0		0
Other invested assets	564,702		540,409
Total investments	809,739		884,358
Senior notes	0		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Foreign exchange contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	[1]	0	[1]
Derivative liabilities	0	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Interest rate futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	[1]	0	[1]
Derivative liabilities	0	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments	0		0
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | 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			0
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale fixed maturity investments			0
Trading fixed maturity investments	0		0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	169,270		249,204
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	$ 75,767		$ 94,745

[1] Asset and liability derivatives relating to foreign exchange contracts and interest rate futures are classified within "other assets" or "accounts payable and accrued liabilities" on the consolidated balance sheets.