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Variable Interest Entities - Royalty Trusts - Initial Public Offering (Parenthetical) (Details) (Royalty Trusts)
3 Months Ended
Mar. 31, 2013
Royalty Trusts
 
Variable Interest Entity [Line Items]  
Percentage of royalty interest conveyed to company at trust termination 50.00%
Percentage of royalty interest sold at trust termination 50.00%