Contingencies (Details)			9 Months Ended	12 Months Ended
	Apr. 17, 2015 Insurance_company People Plaintiff Entity	Apr. 17, 2013 People	Sep. 30, 2018 Litigation_case	Dec. 31, 2015 mine
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 [Member]
Loss Contingencies [Line Items]
Loss Contingency, Claims Settled, Number | Litigation_case			200
IDAHO
Loss Contingencies [Line Items]
Loss contingency, number of mines | mine				17