LOSS ON IMPAIRMENT (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Concentration Risk [Line Items]
Asset impairment charges	$ 143,324	$ 0	$ 143,324	$ 0
Land, Mineral Rights, Property, Plant and Equipment, Net
Concentration Risk [Line Items]
Asset impairment charges	109,979		109,979
Advanced Coal Royalties
Concentration Risk [Line Items]
Asset impairment charges	7,864		7,864
Other Assets
Concentration Risk [Line Items]
Asset impairment charges	25,481		25,481
Coal - U.S. Segment
Concentration Risk [Line Items]
Asset impairment charges	65,649		65,649
Coal - U.S. Segment | Land, Mineral Rights, Property, Plant and Equipment, Net
Concentration Risk [Line Items]
Asset impairment charges	59,262		59,262
Coal - U.S. Segment | Advanced Coal Royalties
Concentration Risk [Line Items]
Asset impairment charges	4,719		4,719
Coal - U.S. Segment | Other Assets
Concentration Risk [Line Items]
Asset impairment charges	1,668		1,668
Coal - WMLP Segment
Concentration Risk [Line Items]
Asset impairment charges	77,675		77,675
Coal - WMLP Segment | Land, Mineral Rights, Property, Plant and Equipment, Net
Concentration Risk [Line Items]
Asset impairment charges	50,717		50,717
Coal - WMLP Segment | Advanced Coal Royalties
Concentration Risk [Line Items]
Asset impairment charges	3,145		3,145
Coal - WMLP Segment | Other Assets
Concentration Risk [Line Items]
Asset impairment charges	$ 23,813		$ 23,813