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Variable Interest Entities - Royalty Trusts - Distributions (Details) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Variable Interest Entity [Line Items]      
Distributions to third-party unitholders $ 181,727,000 $ 60,200,000 $ 3,515,000
Royalty Trusts
      
Variable Interest Entity [Line Items]      
Total distributions 275,000,000 91,200,000  
Distributions to third-party unitholders $ 181,700,000 $ 57,400,000