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Variable Interest Entities - Royalty Trusts - Common and Subordinated Units Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Mississippian Trust I
Variable Interest Entity [Line Items]
Total outstanding common units (in shares)	28,000,000	28,000,000
Total outstanding subordinated units (in shares)	0	0
Permian Trust
Variable Interest Entity [Line Items]
Total outstanding common units (in shares)	39,375,000	39,375,000
Total outstanding subordinated units (in shares)	13,125,000	13,125,000
Mississippian Trust II
Variable Interest Entity [Line Items]
Total outstanding common units (in shares)	37,293,750	37,293,750
Total outstanding subordinated units (in shares)	12,431,250	12,431,250