Principles of Consolidation and Nature of Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Subsidiaries
Variable Interest Entity [Line Items]
Number of wholly owned subsidiaries	2
Variable interest entity, ownership percentage by parent	100.00%
Coyote Creek
Variable Interest Entity [Line Items]
Production capacity, mid 2016	2,500,000
NACoal
Variable Interest Entity [Line Items]
Provision for doubtful accounts	1.1
Minimum | Camino Real
Variable Interest Entity [Line Items]
Production capacity in 2015	2,500,000
Maximum | Camino Real
Variable Interest Entity [Line Items]
Production capacity in 2015	3,000,000