v2.4.0.6
Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments $ 4,557,139 $ 4,748,863
Short-term investments 997,889 821,163
Financial institution securities 555 58,186
Other investments 652,802 644,711
Total other assets and (liabilities) 22,958 29,085
Total assets and liabilities measured on recurring basis 6,231,343 6,302,008
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 1,186,742 1,259,800
Short-term investments 0 0
Financial institution securities 555 58,186
Other investments 0 0
Total other assets and (liabilities) 399 (125)
Total assets and liabilities measured on recurring basis 1,187,696 1,317,861
Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 3,342,330 3,461,271
Short-term investments 997,889 821,163
Financial institution securities 0 0
Other investments 250,596 263,644
Total other assets and (liabilities) 1,231 3,270
Total assets and liabilities measured on recurring basis 4,592,046 4,549,348
Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 28,067 27,792
Short-term investments 0 0
Financial institution securities 0 0
Other investments 402,206 381,067
Total other assets and (liabilities) 21,328 25,940
Total assets and liabilities measured on recurring basis 451,601 434,799
U.S. treasuries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   1,259,800
U.S. treasuries [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]    
Fixed maturity investments 1,186,742 1,259,800
U.S. treasuries [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
U.S. treasuries [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Agencies [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   315,154
Agencies [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]    
Fixed maturity investments 0 0
Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 285,969 315,154
Agencies [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Non-U.S. government (Sovereign Debt) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   133,198
Non-U.S. government (Sovereign Debt) [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]    
Fixed maturity investments 0 0
Non-U.S. government (Sovereign Debt) [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 199,561 133,198
Non-U.S. government (Sovereign Debt) [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Non-U.S. government-backed corporate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   349,514
Non-U.S. government-backed corporate [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]    
Fixed maturity investments 0 0
Non-U.S. government-backed corporate [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 291,077 349,514
Non-U.S. government-backed corporate [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Corporate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   1,615,207
Corporate [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]    
Fixed maturity investments 0 0
Corporate [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 1,575,504 1,587,415
Corporate [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 28,067 27,792
Agency mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   408,531
Agency mortgage-backed [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]    
Fixed maturity investments 0 0
Agency mortgage-backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 440,538 408,531
Agency mortgage-backed [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Non-agency mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   248,339
Non-agency mortgage-backed [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]    
Fixed maturity investments 0 0
Non-agency mortgage-backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 240,217 248,339
Non-agency mortgage-backed [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Commercial mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   406,166
Commercial mortgage-backed [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]    
Fixed maturity investments 0 0
Commercial mortgage-backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 297,101 406,166
Commercial mortgage-backed [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments   12,954
Asset-backed [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]    
Fixed maturity investments 0 0
Asset-backed [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 12,363 12,954
Asset-backed [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 0 0
Private equity partnerships [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   344,669
Private equity partnerships [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]    
Other investments 0 0
Private equity partnerships [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Private equity partnerships [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   344,669
Senior secured bank loan funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   202,929
Senior secured bank loan funds [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]    
Other investments 0 0
Senior secured bank loan funds [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 175,577 172,334
Senior secured bank loan funds [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   30,595
Catastrophe Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   91,310
Catastrophe Bonds [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]    
Other investments 0 0
Catastrophe Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 75,019 91,310
Catastrophe Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Hedge funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   5,803
Hedge funds [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]    
Other investments 0 0
Hedge funds [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Hedge funds [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   5,803
Miscellaneous other investments [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]    
Other investments 0  
Miscellaneous other investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0  
Miscellaneous other investments [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 22,784  
Assumed and ceded reinsurance contracts [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 676 2,647
Assumed and ceded reinsurance contracts [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]    
Total other assets and (liabilities) 0 0
Assumed and ceded reinsurance contracts [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 0 0
Assumed and ceded reinsurance contracts [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities)   2,647
Derivatives [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 23,663 [1] 19,123 [1]
Derivatives [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]    
Total other assets and (liabilities) 399 [1] (125) [1]
Derivatives [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 3,237 [1] 14,821 [1]
Derivatives [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 20,027 [1] 4,427 [1]
Other [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) (1,381) 7,315
Other [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]    
Total other assets and (liabilities) 0 0
Other [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) (2,006) (11,551)
Other [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 625 18,866
Private Equity Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 345,666  
Senior secured bank loan funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 204,114  
Catastrophe Bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 75,019  
Hedge funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 5,219  
Miscellaneous other investments [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 22,784  
U.S. treasuries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 1,186,742  
Agencies [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 285,969  
Non-U.S. government (Sovereign Debt) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 199,561  
Non-U.S. government-backed corporate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 291,077  
Corporate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 1,603,571  
Agency mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 440,538  
Non-agency mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 240,217  
Commercial mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 297,101  
Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments 12,363  
Hedge funds [Member] | Net Asset Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 5,219  
Private Equity Funds [Member] | Net Asset Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 345,666  
Senior secured bank loan funds [Member] | Net Asset Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments $ 28,537  
[1] See "Note 11. Derivative Instruments" for additional information related to the fair value by type of contract, of derivatives entered into by the Company.