SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segments	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segments			5
Revenues	$ 276,312	$ 327,314	$ 571,989	$ 674,326
Depreciation, depletion and amortization	26,396	39,497	52,431	76,064
Operating income (loss)	(157,985)	(13,042)	(147,558)	(13,115)
Total assets	1,457,595	1,670,219	1,457,595	1,670,219	$ 1,663,918
Capital expenditures	5,609	5,894	11,773	13,104
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	102,492	145,642	221,090	288,100
Depreciation, depletion and amortization	10,007	22,644	20,344	38,643
Operating income (loss)	(65,529)	(1,247)	(56,394)	8,991
Total assets	715,274	770,754	715,274	770,754
Capital expenditures	892	1,961	1,643	3,856
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues	109,525	89,033	218,797	200,233
Depreciation, depletion and amortization	7,186	6,782	14,704	17,036
Operating income (loss)	14,847	(12,078)	33,947	(17,052)
Total assets	461,676	435,820	461,676	435,820
Capital expenditures	3,088	1,956	6,979	4,050
Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues	64,295	81,052	132,102	155,857
Depreciation, depletion and amortization	9,189	10,111	17,354	20,461
Operating income (loss)	(82,471)	7,588	(84,468)	8,870
Total assets	236,770	370,936	236,770	370,936
Capital expenditures	1,629	1,977	3,151	5,198
Power Segment
Segment Reporting Information [Line Items]
Revenues	0	19,880	0	41,107
Depreciation, depletion and amortization	0	0	0	0
Operating income (loss)	0	(383)	0	(1,136)
Total assets	0	64,269	0	64,269
Capital expenditures	0	0	0	0
Heritage Segment
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Depreciation, depletion and amortization	0	0	0	0
Operating income (loss)	(1,287)	(1,474)	(2,514)	(2,833)
Total assets	17,583	16,699	17,583	16,699
Capital expenditures	0	0	0	0
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues	0	(8,293)	0	(10,971)
Depreciation, depletion and amortization	14	(40)	29	(76)
Operating income (loss)	(23,545)	(5,448)	(38,129)	(9,955)
Total assets	26,292	11,741	26,292	11,741
Capital expenditures	$ 0	$ 0	$ 0	$ 0