Recently Issued Accounting Standards - Disaggregation of Revenue (Details) - NACoal - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 31,200	$ 28,300
Consolidated operations - long-term contracts
Disaggregation of Revenue [Line Items]
Revenues	28,023	25,201
Royalty
Disaggregation of Revenue [Line Items]
Revenues	3,177	3,099
Goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	17,021	16,915
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	$ 14,179	$ 11,385