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Investments (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost $ 518,827 $ 424,390
Investment Owned, at Fair Value 514,111 407,637
Investment Owned Carrying Value 514,111 407,637
Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 58,017 116,393
Investment Owned, at Fair Value 53,301 99,640
Investment Owned Carrying Value 53,301 99,640
Cash and cash equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 365,675 229,685
Investment Owned, at Fair Value 365,675 229,685
Investment Owned Carrying Value 365,675 229,685
Restricted cash and cash equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 95,135 78,312
Investment Owned, at Fair Value 95,135 78,312
Investment Owned Carrying Value 95,135 78,312
U.S. government obligations and agencies [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 3,193 3,179
Investment Owned, at Fair Value 4,008 3,801
Investment Owned Carrying Value 4,008 3,801
Equity Securities Common Stock Metals and mining [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 11,709 50,121
Investment Owned, at Fair Value 9,235 38,816
Investment Owned Carrying Value 9,235 38,816
Equity Securities Common Stock Energy [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 9,968 6,077
Investment Owned, at Fair Value 8,067 4,999
Investment Owned Carrying Value 8,067 4,999
Equity Securities Common Stock Other [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 10,359 8,044
Investment Owned, at Fair Value 9,989 6,945
Investment Owned Carrying Value 9,989 6,945
Equity Securities Exchange Traded and Mutual Funds Metals and mining [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 6,297 28,311
Investment Owned, at Fair Value 6,527 25,997
Investment Owned Carrying Value 6,527 25,997
Equity Securities Exchange Traded and Mutual Funds Agriculture [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 7,046 17,781
Investment Owned, at Fair Value 7,015 16,878
Investment Owned Carrying Value 7,015 16,878
Equity Securities Exchange Traded and Mutual Funds Energy [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 2,880  
Investment Owned, at Fair Value 2,935  
Investment Owned Carrying Value 2,935  
Equity Securities Exchange Traded and Mutual Funds Indices [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 5,774 2,006
Investment Owned, at Fair Value 5,107 1,710
Investment Owned Carrying Value 5,107 1,710
Non-hedging derivative asset [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 274 357
Investment Owned, at Fair Value 101 123
Investment Owned Carrying Value 101 123
Other investments [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 517 517
Investment Owned, at Fair Value 317 371
Investment Owned Carrying Value $ 317 $ 371