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
Jun. 30, 2012
Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments $ 5,057,057 $ 4,433,517
Short-term investments 654,912 905,477
Equity investments trading 55,381 50,560
Other investments 743,568 748,984
Total other assets and (liabilities) 35,252 16,071
Total assets and liabilities measured on recurring basis 6,546,170 6,154,609
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 fixed maturity investments 1,126,759 885,152
Short-term investments 0 0
Equity investments trading 55,381 50,560
Other investments 0 0
Total other assets and (liabilities) (2,593) (6,162)
Total assets and liabilities measured on recurring basis 1,179,547 929,550
Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 3,902,523 3,520,604
Short-term investments 654,912 905,477
Equity investments trading 0 0
Other investments 353,063 352,458
Total other assets and (liabilities) 3,660 (6,293)
Total assets and liabilities measured on recurring basis 4,914,158 4,772,246
Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 27,775 27,761
Short-term investments 0 0
Equity investments trading 0 0
Other investments 390,505 396,526
Total other assets and (liabilities) 34,185 28,526
Assets (Liabilities), Fair Value Disclosure, Recurring 34,185  
Total assets and liabilities measured on recurring basis 452,465 452,813
U.S. treasuries [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 1,126,759 885,152
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]    
Total fixed maturity investments 1,126,759 885,152
U.S. treasuries [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
U.S. treasuries [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
Agencies [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 436,691 158,561
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]    
Total 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]    
Total fixed maturity investments 436,691 158,561
Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 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]    
Total fixed maturity investments 177,231 227,912
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]    
Total 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]    
Total fixed maturity investments 177,231 227,912
Non-U.S. government (Sovereign Debt) [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
FDIC guaranteed corporate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 29,386 423,630
FDIC guaranteed 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]    
Total fixed maturity investments 0 0
FDIC guaranteed corporate [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 29,386 423,630
FDIC guaranteed corporate [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 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]    
Total fixed maturity investments 393,875 641,082
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]    
Total 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]    
Total fixed maturity investments 393,875 641,082
Non-U.S. government-backed corporate [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
Corporate [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 1,664,722 1,206,904
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]    
Total 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]    
Total fixed maturity investments 1,636,947 1,179,143
Corporate [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 27,775 27,761
Agency mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 592,355 441,749
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]    
Total 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]    
Total fixed maturity investments 592,355 441,749
Agency mortgage-backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
Non-agency mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 153,674 104,771
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]    
Total 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]    
Total fixed maturity investments 153,674 104,771
Non-agency mortgage-backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
Commercial mortgage-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 470,167 325,729
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]    
Total 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]    
Total fixed maturity investments 470,167 325,729
Commercial mortgage-backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
Asset-backed [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 12,197 18,027
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]    
Total 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]    
Total fixed maturity investments 12,197 18,027
Asset-backed [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fixed maturity investments 0 0
Private equity partnerships [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 360,268 367,909
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] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 360,268 367,909
Senior secured bank loan funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 269,317 257,870
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 246,593 237,815
Senior secured bank loan funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 22,724 20,055
Catastrophe bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 106,470 70,999
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 106,470 70,999
Catastrophe bonds [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Non-U.S. fixed income funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   28,862
Non-U.S. fixed income 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
Non-U.S. fixed income funds [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   28,862
Non-U.S. fixed income funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments   0
Hedge funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 6,243 21,344
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 14,782
Hedge funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 6,243 6,562
Miscellaneous other investments [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,270 2,000
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 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 0
Miscellaneous other investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,270 2,000
Assumed and ceded reinsurance contracts [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 9,689 2,115
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] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 9,689 2,115
Derivatives [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 14,752 [1] 3,312 [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) 2,011 707 [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) 3,660 (6,293) [1]
Derivatives [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 9,081 [1] 8,898 [1]
Other [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) 10,811 10,644
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) (4,604) (6,869)
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) 0 0
Other [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total other assets and (liabilities) $ 15,415 $ 17,513
[1] See "Note 12. Derivative Instruments" for additional information related to the fair value by type of contract, of derivatives entered into by the Company.