Commitments and Contingencies - Legal Matters (Details) $ in Millions		1 Months Ended				6 Months Ended		12 Months Ended
	Feb. 29, 2012 USD ($) plaintiff defendant	Mar. 31, 2016 plaintiff	Sep. 30, 2015 USD ($) defendant	Aug. 31, 2015 USD ($)	Aug. 31, 2014 plaintiff defendant	Mar. 31, 2017 USD ($) plaintiff	Mar. 31, 2016 USD ($)	Sep. 30, 2010 item
Commitments and Contingencies
Long term capital commitments on property, plant and equipment						$ 5.2
Legal Matters
Commitments and Contingencies
Legal fees						1.0
Legal fees net of settlement of certain litigation							$ (2.1)
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
Number of defendants | defendant	15
Damages for removal and remediation of fly ash and water supply	$ 850.0
Damages for vexation	1,900.0
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
Number of insurers who have denied coverage | item								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 plaintiffs | plaintiff		7				1
Loss of Enjoyment of Property | Fentress Families Trust
Commitments and Contingencies
Number of plaintiffs | plaintiff						74