Recently Issued Accounting Standards - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 31,440	$ 21,941	$ 96,321	$ 78,341
NACoal
Disaggregation of Revenue [Line Items]
Revenues	31,440	21,941	96,321	78,341
NACoal | Operating segments
Disaggregation of Revenue [Line Items]
Revenues			96,321	78,341
Consolidated operations - long-term contracts | NACoal
Disaggregation of Revenue [Line Items]
Revenues	27,569	19,318	85,066	69,397
Royalty | NACoal
Disaggregation of Revenue [Line Items]
Revenues	3,871	2,623	11,255	8,944
Goods transferred at a point in time | NACoal
Disaggregation of Revenue [Line Items]
Revenues	17,975	12,144	55,170	46,551
Services transferred over time | NACoal
Disaggregation of Revenue [Line Items]
Revenues	$ 13,465	$ 9,797	$ 41,151	$ 31,790