Supplemental Information Relating to Oil and Natural Gas Producing Activities (Unaudited) - Costs Incurred in Oil and Gas Property Acquisiton, Exploration, and Development (Details) (Compass, USD $) In Millions, unless otherwise specified	7 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2014
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Proved property acquisition costs	$ 569.5	$ 0
Unproved property acquisition costs	53.9	0
Total property acquisition costs	623.4	0
Development	11.8	11.4
Lease acquisitions and other	0	0.2
Capitalized asset retirement costs	$ 0.1	$ 0.1
Depreciation, depletion and amortization per unit	1.67	1.45
East Texas/North Louisiana
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates (Energy), Other Factors	7,500,000,000.0	11.5