Self Insurance (Details) $ in Millions	Dec. 31, 2015 item mi	Aug. 31, 2010 USD ($)
Property, Plant and Equipment [Line Items]
Area of self-insured mining units | mi	20
Mining equipment at historical cost | $		$ 250
High Efficiency Units [Member]
Property, Plant and Equipment [Line Items]
Number of mining units with self-insured equipment | item	10