Self Insurance (Details) $ in Millions	Dec. 31, 2016 item mi	Aug. 31, 2010 USD ($)
Property, Plant and Equipment [Line Items]
Area of self-insured mining units | mi	18
Mining equipment at historical cost | $		$ 248
High Efficiency Units [Member]
Property, Plant and Equipment [Line Items]
Number of mining units with self-insured equipment | item	10