Principles of Consolidation and Nature of Operations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 Subsidiaries T	Dec. 31, 2014 USD ($)
Variable Interest Entity [Line Items]
Number of wholly owned subsidiaries | Subsidiaries	2
Variable interest entity, ownership percentage by parent	100.00%
Liberty
Variable Interest Entity [Line Items]
Production capacity in 2023	4,600,000
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
Variable Interest Entity [Line Items]
Annual coal delivery commitment	5,000,000.0
Minimum | Camino Real
Variable Interest Entity [Line Items]
Production capacity in 2015	2,500,000
Maximum
Variable Interest Entity [Line Items]
Annual coal delivery commitment	6,000,000.0
Maximum | Camino Real
Variable Interest Entity [Line Items]
Production capacity in 2015	2,700,000