Self Insurance (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 site property mi	Aug. 31, 2010
Property, Plant and Equipment [Line Items]
Number of mines with self-insured mining equipment	3
Area of self-insured mining units	30
Number of locations with self-insured mining equipment	11
Mining equipment at historical cost		$ 246
High Efficiency Units [Member]
Property, Plant and Equipment [Line Items]
Number of mining units with self-insured equipment	11