Fair Value of Financial Instruments (Schedule of Fair Value of Financial Instruments) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Senior notes	$ 797,823	$ 797,700
U.S. Government and Government Agencies [Member] | Carrying Amount [Member]
Available for sale securities	39,016
Trading securities	982,274
U.S. Government and Government Agencies [Member] | Fair Value Fair Value Disclosure [Member]
Available for sale securities	39,016
Trading securities	982,274
U.S. Government and Government Agencies [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available for sale securities	39,016
Trading securities	835,748
U.S. Government and Government Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member]
Available for sale securities
Trading securities	146,526
U.S. Government and Government Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
Available for sale securities
Trading securities
Non-U.S. Government and Government Agencies [Member] | Carrying Amount [Member]
Trading securities	177,529
Non-U.S. Government and Government Agencies [Member] | Fair Value Fair Value Disclosure [Member]
Trading securities	177,529
Non-U.S. Government and Government Agencies [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Trading securities
Non-U.S. Government and Government Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member]
Trading securities	177,529
Non-U.S. Government and Government Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
Trading securities
States, Municipalities and Political Subdivisions [Member] | Carrying Amount [Member]
Available for sale securities	48,666
Trading securities	108,256
States, Municipalities and Political Subdivisions [Member] | Fair Value Fair Value Disclosure [Member]
Available for sale securities	48,666
Trading securities	108,256
States, Municipalities and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available for sale securities
Trading securities
States, Municipalities and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Available for sale securities	48,666
Trading securities	108,256
States, Municipalities and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Available for sale securities
Trading securities
Corporate Debt [Member] | Carrying Amount [Member]
Available for sale securities	257,869
Trading securities	2,348,875
Corporate Debt [Member] | Fair Value Fair Value Disclosure [Member]
Available for sale securities	257,869
Trading securities	2,348,875
Corporate Debt [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available for sale securities
Trading securities
Corporate Debt [Member] | Significant Other Observable Inputs (Level 2) [Member]
Available for sale securities	257,869
Trading securities	2,348,875
Corporate Debt [Member] | Significant Unobservable Inputs (Level 3) [Member]
Available for sale securities
Trading securities
Mortgage-Backed [Member] | Carrying Amount [Member]
Trading securities	1,904,564
Mortgage-Backed [Member] | Fair Value Fair Value Disclosure [Member]
Trading securities	1,904,564
Mortgage-Backed [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Trading securities
Mortgage-Backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
Trading securities	1,687,904
Mortgage-Backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
Trading securities	216,660
Asset-Backed [Member] | Carrying Amount [Member]
Trading securities	679,536
Asset-Backed [Member] | Fair Value Fair Value Disclosure [Member]
Trading securities	679,536
Asset-Backed [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Trading securities
Asset-Backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
Trading securities	566,226
Asset-Backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
Trading securities	113,310
Hedge Funds [Member] | Carrying Amount [Member]
Hedge funds	562,267
Hedge Funds [Member] | Fair Value Fair Value Disclosure [Member]
Hedge funds	562,267
Hedge Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Hedge funds
Hedge Funds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Hedge funds
Hedge Funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Hedge funds	562,267
Equity Securities [Member] | Carrying Amount [Member]
Equity securities	393,913
Equity Securities [Member] | Fair Value Fair Value Disclosure [Member]
Equity securities	393,913
Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Equity securities	393,913
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Equity securities
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Equity securities
Carrying Amount [Member]
Available for sale securities	345,551
Trading securities	6,201,034
Total fixed maturity investments	6,546,585
Total investments	7,502,765
Senior notes	797,823
Fair Value Fair Value Disclosure [Member]
Available for sale securities	345,551
Trading securities	6,201,034
Total fixed maturity investments	6,546,585
Total investments	7,502,765
Senior notes	878,085
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available for sale securities
Trading securities
Total fixed maturity investments
Total investments	1,268,677
Senior notes
Significant Other Observable Inputs (Level 2) [Member]
Available for sale securities
Trading securities
Total fixed maturity investments
Total investments	5,341,851
Senior notes	878,085
Significant Unobservable Inputs (Level 3) [Member]
Available for sale securities
Trading securities
Total fixed maturity investments
Total investments	892,237
Senior notes