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Investments (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost $ 499,497 $ 476,972
Investment Owned, at Fair Value 499,497 469,747
Investment Owned, at Carrying Value 499,497 469,747
Categories of Investments, Marketable Securities, Trading Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost    96,571
Investment Owned, at Fair Value    89,346
Investment Owned, at Carrying Value    89,346
Categories of Investments, Marketable Securities, Available-for-sale Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 13,968   
Investment Owned, at Fair Value 13,968   
Investment Owned, at Carrying Value 13,968   
Cash and cash equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 482,876 347,392
Investment Owned, at Fair Value 482,876 347,392
Investment Owned, at Carrying Value 482,876 347,392
Restricted cash and cash equivalents [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 2,653 33,009
Investment Owned, at Fair Value 2,653 33,009
Investment Owned, at Carrying Value 2,653 33,009
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
Investment Owned, at Fair Value    4,009
Investment Owned, at Carrying Value    4,009
U.S. government obligations and agencies [Member] | Categories of Investments, Marketable Securities, Available-for-sale Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 8,004   
Investment Owned, at Fair Value 8,013   
Investment Owned, at Carrying Value 8,013   
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    31,113
Investment Owned, at Fair Value    26,130
Investment Owned, at Carrying Value    26,130
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    12,053
Investment Owned, at Fair Value    10,868
Investment Owned, at Carrying Value    10,868
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    8,416
Investment Owned, at Fair Value    8,215
Investment Owned, at Carrying Value    8,215
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    22,687
Investment Owned, at Fair Value    21,989
Investment Owned, at Carrying Value    21,989
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    10,705
Investment Owned, at Fair Value    10,265
Investment Owned, at Carrying Value    10,265
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    4,992
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    2,827
Investment Owned, at Fair Value    2,506
Investment Owned, at Carrying Value    2,506
Non-hedging derivative 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
Investment Owned, at Fair Value    (21)
Investment Owned, at Carrying Value    (21)
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
Investment Owned, at Fair Value    317
Investment Owned, at Carrying Value    317
Corporate bonds [Member] | Categories of Investments, Marketable Securities, Available-for-sale Securities [Member]
    
Investment holdings by type of instrument    
Investment Owned, at Cost or Amortized Cost 5,964   
Investment Owned, at Fair Value 5,955   
Investment Owned, at Carrying Value $ 5,955