Commitments and Contingencies - Legal Matters (Details) $ in Millions	1 Months Ended				12 Months Ended
	Sep. 30, 2015 USD ($) defendant	Aug. 31, 2015 USD ($)	Aug. 31, 2014 plaintiff defendant	Feb. 29, 2012 USD ($) plaintiff defendant	Sep. 30, 2016 USD ($) plaintiff	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2010 item
Commitments and Contingencies
Long term capital commitments on property, plant and equipment					$ 3.6
Amount accrued for potential losses							$ 2.8
Legal Matters
Commitments and Contingencies
Legal fees						$ 2.2	$ 5.1
Legal fees net of settlement of certain litigation					(1.4)
Total liability accrued					0.0
Legal Matters | Minimum
Commitments and Contingencies
Potential loss for unresolved matters					$ 0.0
Fentress Families Trust
Commitments and Contingencies
Number of plaintiffs | plaintiff				383	7
Number of defendants | defendant				15
Damages for removal and remediation of fly ash and water supply				$ 850.0
Damages for vexation				1.9
Damages for others				8.0
Damages for properties plus prejudgment interest, attorney fees and costs				55.0
Additional damages sought by other plaintiffs				$ 307.2
CPM Virginia LLC
Commitments and Contingencies
Loss contingency damages sought value	$ 0.5	$ 840.0
Clary
Commitments and Contingencies
Number of plaintiffs | plaintiff			77
Number of defendants | defendant			4
Ohio Power Company
Commitments and Contingencies
Number of defendants | defendant	2
Personal Injury Claims | Fentress Families Trust
Commitments and Contingencies
Number of insurers who have denied coverage | item								2