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Investments (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost $ 469,453 $ 424,390
Investment Owned, at Fair Value 469,747 407,637
Investment Owned, at Carrying Value 469,747 407,637
Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 89,052 116,393
Investment Owned, at Fair Value 89,346 99,640
Investment Owned, at Carrying Value 89,346 99,640
Cash and cash equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 347,392 229,685
Investment Owned, at Fair Value 347,392 229,685
Investment Owned, at Carrying Value 347,392 229,685
Restricted cash and cash equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 33,009 78,312
Investment Owned, at Fair Value 33,009 78,312
Investment Owned, at Carrying Value 33,009 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 3,192 3,179
Investment Owned, at Fair Value 4,009 3,801
Investment Owned, at Carrying Value 4,009 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 26,630 50,121
Investment Owned, at Fair Value 26,130 38,816
Investment Owned, at Carrying Value 26,130 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 10,914 6,077
Investment Owned, at Fair Value 10,868 4,999
Investment Owned, at Carrying Value 10,868 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 8,152 8,044
Investment Owned, at Fair Value 8,215 6,945
Investment Owned, at Carrying Value 8,215 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 21,748 28,311
Investment Owned, at Fair Value 21,989 25,997
Investment Owned, at Carrying Value 21,989 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 10,303 17,781
Investment Owned, at Fair Value 10,265 16,878
Investment Owned, at Carrying Value 10,265 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 4,939   
Investment Owned, at Fair Value 5,068   
Investment Owned, at Carrying Value 5,068   
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 2,588 2,006
Investment Owned, at Fair Value 2,506 1,710
Investment Owned, at Carrying Value 2,506 1,710
Non- Hedging Derivative Instrument Asset Liability Net [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 69 357
Investment Owned, at Fair Value (21) 123
Investment Owned, at Carrying Value (21) 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, at Carrying Value $ 317 $ 371