Segment Information - Reconciliation of Net Income (Loss) to Adjusted EBITDA (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net loss from continuing operations	$ (68,637,000)	$ (43,561,000)	$ (346,746,000)	$ (11,271,000)
Interest expense	18,389,000	18,847,000	54,808,000	50,079,000
Interest income	(378,000)	(1,763,000)	(6,889,000)	(5,584,000)
Income tax expense (benefit)	(45,000)	(3,102,000)	(2,200,000)	(8,880,000)
Depreciation, depletion and amortization	50,739,000	60,842,000	154,466,000	184,927,000
Merger-related costs	0	68,000	0	1,055,000
Non-cash stock compensation expense	1,078,000	2,738,000	4,200,000	7,463,000
Mark-to-market adjustment - acquisition-related obligations		(3,238,000)	(13,425,000)	(288,000)
Mark-to-market adjustment for acquisition-related obligations	3,624,000	(3,238,000)	(13,425,000)	(288,000)
Accretion on asset retirement obligations	9,127,000	6,846,000	23,806,000	19,925,000
Loss on modification and extinguishment of debt	0	0	0	26,459,000
Asset impairment	3,516,000	32,000	198,963,000	5,858,000
Asset impairment and restructuring			221,453,000	23,020,000
Loss on partial settlement of benefit obligations			1,230,000
Cost impact of coal inventory fair value adjustment				8,209,000
Gain on assets acquired in an exchange transaction			0	(9,083,000)
Amortization of acquired intangibles, net	2,219,000	2,314,000	5,180,000	(4,712,000)
Adjusted EBITDA	19,687,000	40,023,000	96,830,000	264,157,000
Restructuring expense		0		0
Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	3,571,000		221,453,000
Management restructuring costs	55,000		20,608,000
Restructuring expense	55,000		22,490,000
Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			947,000
Operating Segments | CAPP - Met
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net loss from continuing operations	(29,545,000)	13,033,000	(67,996,000)	154,020,000
Interest expense	(189,000)	84,000	(1,495,000)	306,000
Interest income	(3,000)	(4,000)	(63,000)	(12,000)
Income tax expense (benefit)	0	0	0	0
Depreciation, depletion and amortization	41,177,000	38,212,000	121,699,000	113,714,000
Merger-related costs		0		0
Non-cash stock compensation expense	105,000	348,000	410,000	1,127,000
Mark-to-market adjustment - acquisition-related obligations		0	0	0
Mark-to-market adjustment for acquisition-related obligations	0
Accretion on asset retirement obligations	3,767,000	2,326,000	10,786,000	6,986,000
Loss on modification and extinguishment of debt				0
Asset impairment		32,000		5,858,000
Loss on partial settlement of benefit obligations			0
Cost impact of coal inventory fair value adjustment				4,751,000
Gain on assets acquired in an exchange transaction				(9,083,000)
Amortization of acquired intangibles, net	2,535,000	4,765,000	7,875,000	5,816,000
Adjusted EBITDA	17,847,000	58,796,000	104,668,000	283,483,000
Operating Segments | CAPP - Met | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	0		32,951,000
Operating Segments | CAPP - Met | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			501,000
Operating Segments | CAPP - Thermal
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net loss from continuing operations	(3,815,000)	(11,338,000)	(37,711,000)	(34,675,000)
Interest expense	2,000	6,000	6,000	16,000
Interest income	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Depreciation, depletion and amortization	7,313,000	13,972,000	19,422,000	44,586,000
Merger-related costs		0		0
Non-cash stock compensation expense	0	3,000	8,000	60,000
Mark-to-market adjustment - acquisition-related obligations		0	0	0
Mark-to-market adjustment for acquisition-related obligations	0
Accretion on asset retirement obligations	2,406,000	2,670,000	7,025,000	7,401,000
Loss on modification and extinguishment of debt				0
Asset impairment		0		0
Loss on partial settlement of benefit obligations			0
Cost impact of coal inventory fair value adjustment				3,458,000
Gain on assets acquired in an exchange transaction				0
Amortization of acquired intangibles, net	(486,000)	(3,359,000)	(3,484,000)	(12,142,000)
Adjusted EBITDA	5,155,000	1,954,000	5,459,000	8,704,000
Operating Segments | CAPP - Thermal | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	(265,000)		20,188,000
Restructuring expense	(484,000)		1,826,000
Operating Segments | CAPP - Thermal | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			5,000
Operating Segments | NAPP
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net loss from continuing operations	149,000	(12,304,000)	(158,983,000)	(2,118,000)
Interest expense	(357,000)	1,000	(1,431,000)	2,000
Interest income	(1,000)	(15,000)	(15,000)	(38,000)
Income tax expense (benefit)	0	0	0	0
Depreciation, depletion and amortization	1,504,000	6,241,000	10,525,000	19,390,000
Merger-related costs		0		0
Non-cash stock compensation expense	0	0	0	0
Mark-to-market adjustment - acquisition-related obligations		0	0	0
Mark-to-market adjustment for acquisition-related obligations	0
Accretion on asset retirement obligations	2,424,000	1,017,000	3,963,000	3,050,000
Loss on modification and extinguishment of debt				0
Asset impairment		0		0
Loss on partial settlement of benefit obligations			0
Cost impact of coal inventory fair value adjustment				0
Gain on assets acquired in an exchange transaction				0
Amortization of acquired intangibles, net	145,000	908,000	714,000	1,614,000
Adjusted EBITDA	7,250,000	(4,152,000)	20,645,000	21,900,000
Operating Segments | NAPP | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	3,386,000		165,866,000
Restructuring expense	89,000		18,221,000
Operating Segments | NAPP | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			6,000
All Other
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net loss from continuing operations	(35,426,000)	(32,952,000)	(82,056,000)	(128,498,000)
Interest expense	18,933,000	18,756,000	57,728,000	49,755,000
Interest income	(374,000)	(1,744,000)	(6,811,000)	(5,534,000)
Income tax expense (benefit)	(45,000)	(3,102,000)	(2,200,000)	(8,880,000)
Depreciation, depletion and amortization	745,000	2,417,000	2,820,000	7,237,000
Merger-related costs		68,000		1,055,000
Non-cash stock compensation expense	973,000	2,387,000	3,782,000	6,276,000
Mark-to-market adjustment - acquisition-related obligations		(3,238,000)	(13,425,000)	(288,000)
Mark-to-market adjustment for acquisition-related obligations	3,624,000
Accretion on asset retirement obligations	530,000	833,000	2,032,000	2,488,000
Loss on modification and extinguishment of debt				26,459,000
Asset impairment		0		0
Loss on partial settlement of benefit obligations			1,230,000
Cost impact of coal inventory fair value adjustment				0
Gain on assets acquired in an exchange transaction				0
Amortization of acquired intangibles, net	25,000	0	75,000	0
Adjusted EBITDA	(10,565,000)	$ (16,575,000)	(33,942,000)	$ (49,930,000)
All Other | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	450,000		2,448,000
Restructuring expense	$ 450,000		2,443,000
All Other | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			$ 435,000