SEGMENT INFORMATION - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Coal - U.S. Segment
Disaggregation of Revenue [Line Items]
Total	$ 102,492	$ 137,349	$ 221,090	$ 277,129
Coal - U.S. Segment | Domestic Coal Sales
Disaggregation of Revenue [Line Items]
Total	101,099	134,558	217,501	272,884
Coal - U.S. Segment | Export Coal Sales
Disaggregation of Revenue [Line Items]
Total	0	0	0	0
Coal - U.S. Segment | Power Revenues
Disaggregation of Revenue [Line Items]
Total		0		0
Coal - U.S. Segment | Other Revenues
Disaggregation of Revenue [Line Items]
Total	1,393	2,791	3,589	4,245
Coal - Canada Segment
Disaggregation of Revenue [Line Items]
Total	109,525	89,033	218,797	200,233
Coal - Canada Segment | Domestic Coal Sales
Disaggregation of Revenue [Line Items]
Total	68,232	67,468	148,500	143,395
Coal - Canada Segment | Export Coal Sales
Disaggregation of Revenue [Line Items]
Total	40,684	21,383	69,271	56,163
Coal - Canada Segment | Power Revenues
Disaggregation of Revenue [Line Items]
Total		0		0
Coal - Canada Segment | Other Revenues
Disaggregation of Revenue [Line Items]
Total	609	182	1,026	675
Coal - WMLP Segment
Disaggregation of Revenue [Line Items]
Total	64,295	81,052	132,102	155,857
Coal - WMLP Segment | Domestic Coal Sales
Disaggregation of Revenue [Line Items]
Total	60,032	79,817	126,741	152,925
Coal - WMLP Segment | Export Coal Sales
Disaggregation of Revenue [Line Items]
Total	0	0	0	0
Coal - WMLP Segment | Power Revenues
Disaggregation of Revenue [Line Items]
Total		0		0
Coal - WMLP Segment | Other Revenues
Disaggregation of Revenue [Line Items]
Total	$ 4,263	1,235	$ 5,361	2,932
Power Segment
Disaggregation of Revenue [Line Items]
Total		19,880		41,107
Power Segment | Domestic Coal Sales
Disaggregation of Revenue [Line Items]
Total		0		0
Power Segment | Export Coal Sales
Disaggregation of Revenue [Line Items]
Total		0		0
Power Segment | Power Revenues
Disaggregation of Revenue [Line Items]
Total		19,880		41,107
Power Segment | Other Revenues
Disaggregation of Revenue [Line Items]
Total		$ 0		$ 0