Contingencies (Details)			6 Months Ended	9 Months Ended
	Apr. 17, 2015 Entity Insurance_company People Plaintiff	Apr. 17, 2013 People	Jun. 30, 2016 mine	Sep. 30, 2020 USD ($) Litigation_case
Unfavorable Regulatory Action [Member]
Loss Contingencies [Line Items]
Payments for Legal Settlements | $				$ 550,000
Pending Litigation
Loss Contingencies [Line Items]
Number of people killed		15
Number of people injured		200
Number of plaintiffs | Plaintiff	400
Number of entities that filed claims | Entity	9
Number of people that filed claims	325
Number of insurance companies that filed claims | Insurance_company	80
Settled Litigation
Loss Contingencies [Line Items]
Loss Contingency, Claims Settled, Number | Litigation_case				300
IDAHO
Loss Contingencies [Line Items]
Loss contingency, number of mines | mine			17