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Fair Value (Details) (Fair value measurements - recurring [Member], USD $)
In Thousands
Sep. 30, 2011
Dec. 31, 2010
Assets measured at fair value  
Total assets measured at fair value$ 11,616,918$ 11,156,629
Liabilities measured at fair value  
Total liabilities measured at fair value360,663366,913
Fixed maturities fair value [Member] | Corporate bonds [Member]
  
Assets measured at fair value  
Total assets measured at fair value2,766,6512,714,375
Fixed maturities fair value [Member] | Corporate bonds [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Corporate bonds [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value2,625,6302,560,866
Fixed maturities fair value [Member] | Corporate bonds [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value141,021153,509
Fixed maturities fair value [Member] | Mortgage backed securities [Member]
  
Assets measured at fair value  
Total assets measured at fair value1,545,3211,806,813
Fixed maturities fair value [Member] | Mortgage backed securities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Mortgage backed securities [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value1,545,3211,806,813
Fixed maturities fair value [Member] | Mortgage backed securities [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Municipal bonds [Member]
  
Assets measured at fair value  
Total assets measured at fair value1,422,9431,182,100
Fixed maturities fair value [Member] | Municipal bonds [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Municipal bonds [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value1,422,9431,182,100
Fixed maturities fair value [Member] | Municipal bonds [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Commercial mortgage backed securities [Member]
  
Assets measured at fair value  
Total assets measured at fair value1,118,9051,167,299
Fixed maturities fair value [Member] | Commercial mortgage backed securities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Commercial mortgage backed securities [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value1,118,9051,167,299
Fixed maturities fair value [Member] | Commercial mortgage backed securities [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | US government and government agencies [Member]
  
Assets measured at fair value  
Total assets measured at fair value1,263,665872,149
Fixed maturities fair value [Member] | US government and government agencies [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value1,263,665872,149
Fixed maturities fair value [Member] | US government and government agencies [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | US government and government agencies [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Non-US government securities [Member]
  
Assets measured at fair value  
Total assets measured at fair value837,043732,666
Fixed maturities fair value [Member] | Non-US government securities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Non-US government securities [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value837,043732,666
Fixed maturities fair value [Member] | Non-US government securities [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Asset backed securities [Member]
  
Assets measured at fair value  
Total assets measured at fair value647,705558,032
Fixed maturities fair value [Member] | Asset backed securities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member] | Asset backed securities [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value647,705558,032
Fixed maturities fair value [Member] | Asset backed securities [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Fixed maturities fair value [Member]
  
Assets measured at fair value  
Total assets measured at fair value9,602,2339,033,434
Fixed maturities fair value [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value1,263,665872,149
Fixed maturities fair value [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value8,197,5478,007,776
Fixed maturities fair value [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value141,021153,509
Equity securities [Member]
  
Assets measured at fair value  
Total assets measured at fair value273,213310,194
Equity securities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value273,157297,623
Equity securities [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value5612,571
Equity securities [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Other investments fair value [Member]
  
Assets measured at fair value  
Total assets measured at fair value229,974275,538
Other investments fair value [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value00
Other investments fair value [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value223,974267,680
Other investments fair value [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value6,0007,858
Short-term investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value799,662915,841
Short-term investments [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value771,217859,042
Short-term investments [Member] | Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value28,44556,799
Short-term investments [Member] | Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value00
Corporate bonds [Member] | Fixed maturities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Corporate bonds [Member] | Fixed maturities [Member] | Significant Other Observable Inputs ( Level 2) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value107,215124,969
Corporate bonds [Member] | Fixed maturities [Member] | Significant Unobservable Inputs (Level 3) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Corporate bonds [Member] | Fixed maturities [Member] | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value107,215124,969
Non-US government securities [Member] | Fixed maturities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value0 
Non-US government securities [Member] | Fixed maturities [Member] | Significant Other Observable Inputs ( Level 2) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value15,651 
Non-US government securities [Member] | Fixed maturities [Member] | Significant Unobservable Inputs (Level 3) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value0 
Non-US government securities [Member] | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value15,651 
Fixed maturities [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1) | TALF investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value0 
Fixed maturities [Member] | TALF investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value392,455 
Other investments fair value option [Member] | Quoted Prices in Active Markets for Identical Assets ( Level 1) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Other investments fair value option [Member] | Significant Other Observable Inputs ( Level 2) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value95,7960
Other investments fair value option [Member] | Significant Unobservable Inputs (Level 3) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Other investments fair value option [Member] | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value95,7960
Quoted Prices in Active Markets for Identical Assets ( Level 1)
  
Assets measured at fair value  
Total assets measured at fair value2,408,7582,123,018
Liabilities measured at fair value  
Total liabilities measured at fair value46,52641,143
Quoted Prices in Active Markets for Identical Assets ( Level 1) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value100,71994,204
Quoted Prices in Active Markets for Identical Assets ( Level 1) | Investments fair value option [Member] | Equity securities, other [Member]
  
Assets measured at fair value  
Total assets measured at fair value100,71994,204
Quoted Prices in Active Markets for Identical Assets ( Level 1) | TALF investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value 0
Quoted Prices in Active Markets for Identical Assets ( Level 1) | TALF borrowings market value [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value00
Quoted Prices in Active Markets for Identical Assets ( Level 1) | Securities sold but not yet purchased [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value46,52641,143
Significant Other Observable Inputs ( Level 2)
  
Assets measured at fair value  
Total assets measured at fair value9,061,1398,872,244
Liabilities measured at fair value  
Total liabilities measured at fair value314,137325,770
Significant Other Observable Inputs ( Level 2) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value218,662124,969
Significant Other Observable Inputs ( Level 2) | Investments fair value option [Member] | Equity securities, other [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Significant Other Observable Inputs ( Level 2) | TALF investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value392,455402,449
Significant Other Observable Inputs ( Level 2) | TALF borrowings market value [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value314,137325,770
Significant Other Observable Inputs ( Level 2) | Securities sold but not yet purchased [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value00
Significant Unobservable Inputs (Level 3)
  
Assets measured at fair value  
Total assets measured at fair value147,021161,367
Liabilities measured at fair value  
Total liabilities measured at fair value00
Significant Unobservable Inputs (Level 3) | Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Significant Unobservable Inputs (Level 3) | Investments fair value option [Member] | Equity securities, other [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Significant Unobservable Inputs (Level 3) | TALF investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value00
Significant Unobservable Inputs (Level 3) | TALF borrowings market value [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value00
Significant Unobservable Inputs (Level 3) | Securities sold but not yet purchased [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value00
Investments fair value option [Member]
  
Assets measured at fair value  
Total assets measured at fair value319,381219,173
Investments fair value option [Member] | Equity securities, other [Member]
  
Assets measured at fair value  
Total assets measured at fair value100,71994,204
TALF investments [Member]
  
Assets measured at fair value  
Total assets measured at fair value 402,449
TALF borrowings market value [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value314,137325,770
Securities sold but not yet purchased [Member]
  
Liabilities measured at fair value  
Total liabilities measured at fair value$ 46,526$ 41,143