Contingencies (Details)			3 Months Ended
	Apr. 17, 2015 Insurance_company People Plaintiff Entity	Apr. 17, 2013 People	Mar. 31, 2017 processingFacility mine
Loss Contingencies [Line Items]
Loss contingency, number of processing facilities | processingFacility			3
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
IDAHO
Loss Contingencies [Line Items]
Loss contingency, number of mines | mine			18