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Variable Interest Entities - Royalty Trusts - Common and Subordinated Units Outstanding (Details)	6 Months Ended
	Jun. 30, 2015 shares
Mississippian Trust I
Variable Interest Entity
Total outstanding common units (in shares)	28,000,000
Total outstanding subordinated units (in shares)	0
Permian Trust
Variable Interest Entity
Total outstanding common units (in shares)	39,375,000
Total outstanding subordinated units (in shares)	13,125,000
Mississippian Trust II
Variable Interest Entity
Total outstanding common units (in shares)	37,293,750
Total outstanding subordinated units (in shares)	12,431,250