Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets, Net [Abstract]
Net Balance	$ 43,554	$ 45,678
Amortization of intangible assets	2,123	2,503	$ 2,606
Coal supply agreement
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	84,200	84,200
Accumulated Amortization	(40,646)	(38,522)
Net Balance	43,554	$ 45,678
NACoal
Finite-Lived Intangible Assets, Net [Abstract]
Expected annual amortization expense, 2018	3,000
Expected annual amortization expense, 2019	3,000
Expected annual amortization expense, 2020	3,100
Expected annual amortization expense, 2021	3,100
Expected annual amortization expense, 2022	$ 3,100
Coal supply agreement amortization period	30 years