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Investments (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost $ 529,525 $ 424,390
Investment Owned, at Fair Value 516,426 407,637
Investment Owned Carrying Value 516,426 407,637
Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 98,926 116,393
Investment Owned, at Fair Value 85,827 99,640
Investment Owned Carrying Value 85,827 99,640
Cash and Cash Equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 356,325 229,685
Investment Owned, at Fair Value 356,325 229,685
Investment Owned Carrying Value 356,325 229,685
Restricted Cash and Cash Equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 74,274 78,312
Investment Owned, at Fair Value 74,274 78,312
Investment Owned Carrying Value 74,274 78,312
U.S. Government Securities 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 3,342 3,179
Investment Owned, at Fair Value 3,913 3,801
Investment Owned Carrying Value 3,913 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 30,637 50,121
Investment Owned, at Fair Value 23,546 38,816
Investment Owned Carrying Value 23,546 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 11,854 6,077
Investment Owned, at Fair Value 8,962 4,999
Investment Owned Carrying Value 8,962 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 2,904 8,044
Investment Owned, at Fair Value 2,517 6,945
Investment Owned Carrying Value 2,517 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 24,140 28,311
Investment Owned, at Fair Value 22,437 25,997
Investment Owned Carrying Value 22,437 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 18,761 17,781
Investment Owned, at Fair Value 18,172 16,878
Investment Owned Carrying Value 18,172 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 4,420  
Investment Owned, at Fair Value 4,090  
Investment Owned Carrying Value 4,090  
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 2,378 2,006
Investment Owned, at Fair Value 1,989 1,710
Investment Owned Carrying Value 1,989 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 147 357
Investment Owned, at Fair Value 31 123
Investment Owned Carrying Value 31 123
Non-hedging derivative (liability) [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost (174)  
Investment Owned, at Fair Value (174)  
Investment Owned Carrying Value (174)  
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 344 371
Investment Owned Carrying Value $ 344 $ 371