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Variable Interest Entities - Royalty Trusts - Initial Public Offering (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 1 Months Ended 1 Months Ended 1 Months Ended
Apr. 30, 2011
Mississippian Trust I
Dec. 31, 2013
Mississippian Trust I
Dec. 31, 2012
Mississippian Trust I
Aug. 31, 2011
Permian Trust
Dec. 31, 2013
Permian Trust
Dec. 31, 2012
Permian Trust
Apr. 30, 2012
Mississippian Trust II
Dec. 31, 2013
Mississippian Trust II
Dec. 31, 2012
Mississippian Trust II
Variable Interest Entity [Line Items]                  
Net proceeds of offering (in millions) $ 336,893     $ 580,635     $ 587,087    
Total outstanding common units (in shares) 21,000,000     39,375,000     37,293,750    
Total outstanding subordinated units (in shares) 7,000,000     13,125,000     12,431,250    
Beneficial interest owned by Company 38.40% [1] 26.90% 26.90% 34.30% [1] 28.50% 30.50% 39.90% [1] 37.60% 39.90%
Liquidation date Dec. 31, 2030 [2]     Mar. 31, 2031 [2]     Dec. 31, 2031 [2]    
[1] Subsequent to the initial public offerings, the Company sold common units of the Royalty Trusts it owned in transactions exempt from registration under Rule 144 under the Securities Act. These transactions decreased the Company’s beneficial interests in the Royalty Trusts. See further discussion of the unit sales below.
[2] At the time each Royalty Trust terminates, 50% of the royalty interests conveyed to the Royalty Trust will automatically revert to the Company, and the remaining 50% will be sold with the proceeds distributed to the Royalty Trust unitholders.