Segment Information - Reconciliation of Net Income (Loss) to Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	$ (238,301)	$ 24,300	$ (278,109)	$ 32,290
Interest expense	18,814	16,077	36,419	31,232
Interest income	(5,533)	(1,885)	(6,511)	(3,821)
Income tax expense (benefit)	33	(1,000)	(2,155)	(5,778)
Depreciation, depletion and amortization	49,262	62,814	103,727	124,085
Merger-related costs	0	156	0	987
Non-cash stock compensation expense	1,044	(546)	3,122	4,725
Mark-to-market adjustment - acquisition-related obligations		1,014	(17,049)	2,950
Mark-to-market adjustment for acquisition-related obligations	(2,052)	1,014	(17,049)	2,950
Accretion on asset retirement obligations	7,304	6,847	14,679	13,079
Loss on modification and extinguishment of debt	0	26,459	0	26,459
Asset impairment	161,738	5,826	195,447	5,826
Asset impairment and restructuring			217,882	22,294
Loss on partial settlement of benefit obligations	63		1,230
Cost impact of coal inventory fair value adjustment		1,033		8,209
Gain on assets acquired in an exchange transaction			0	(9,083)
Amortization of acquired intangibles, net	2,096	(343)	2,961	(7,026)
Adjusted EBITDA	16,903	140,752	77,143	224,134
Restructuring expense	22,435	0	22,435	0
Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	184,173		217,882
Management restructuring costs	20,553		20,553
Restructuring expense	22,435		22,435
Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			947
Operating Segments | CAPP - Met
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(26,987)	70,600	(38,451)	140,984
Interest expense	(376)	194	(1,306)	222
Interest income	(2)	(4)	(60)	(8)
Income tax expense (benefit)	0	0	0	0
Depreciation, depletion and amortization	38,800	38,829	80,522	75,502
Merger-related costs		0		0
Non-cash stock compensation expense	(94)	376	305	779
Mark-to-market adjustment - acquisition-related obligations		0	0	0
Mark-to-market adjustment for acquisition-related obligations	0
Accretion on asset retirement obligations	3,517	2,327	7,019	4,660
Loss on modification and extinguishment of debt		0		0
Asset impairment		5,826		5,826
Loss on partial settlement of benefit obligations	0		0
Cost impact of coal inventory fair value adjustment		1,033		4,751
Gain on assets acquired in an exchange transaction				(9,083)
Amortization of acquired intangibles, net	2,759	3,870	5,340	1,050
Adjusted EBITDA	17,617	123,051	86,821	224,683
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
Operating Segments | CAPP - Met | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			501
Operating Segments | CAPP - Thermal
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(25,830)	(3,932)	(33,896)	(23,337)
Interest expense	2	6	4	10
Interest income	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Depreciation, depletion and amortization	7,260	16,502	12,109	30,614
Merger-related costs		0		0
Non-cash stock compensation expense	1	5	8	57
Mark-to-market adjustment - acquisition-related obligations		0	0	0
Mark-to-market adjustment for acquisition-related obligations	0
Accretion on asset retirement obligations	2,267	2,666	4,619	4,731
Loss on modification and extinguishment of debt		0		0
Asset impairment		0		0
Loss on partial settlement of benefit obligations	0		0
Cost impact of coal inventory fair value adjustment		0		3,458
Gain on assets acquired in an exchange transaction				0
Amortization of acquired intangibles, net	(903)	(4,213)	(2,998)	(8,782)
Adjusted EBITDA	2,492	11,034	304	6,751
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	19,695		20,453
Restructuring expense	2,310		2,310
Operating Segments | CAPP - Thermal | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			5
Operating Segments | NAPP
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(156,092)	13,771	(159,132)	10,186
Interest expense	(504)	0	(1,074)	1
Interest income	(4)	(11)	(14)	(23)
Income tax expense (benefit)	0	0	0	0
Depreciation, depletion and amortization	2,172	6,522	9,021	13,149
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	769	1,016	1,539	2,033
Loss on modification and extinguishment of debt		0		0
Asset impairment		0		0
Loss on partial settlement of benefit obligations	0		0
Cost impact of coal inventory fair value adjustment		0		0
Gain on assets acquired in an exchange transaction				0
Amortization of acquired intangibles, net	215	0	569	706
Adjusted EBITDA	9,036	21,298	13,395	26,052
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	162,480		162,480
Restructuring expense	18,132		18,132
Operating Segments | NAPP | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			6
All Other
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(29,392)	(56,139)	(46,630)	(95,543)
Interest expense	19,692	15,877	38,795	30,999
Interest income	(5,527)	(1,870)	(6,437)	(3,790)
Income tax expense (benefit)	33	(1,000)	(2,155)	(5,778)
Depreciation, depletion and amortization	1,030	961	2,075	4,820
Merger-related costs		156		987
Non-cash stock compensation expense	1,137	(927)	2,809	3,889
Mark-to-market adjustment - acquisition-related obligations		1,014	(17,049)	2,950
Mark-to-market adjustment for acquisition-related obligations	(2,052)
Accretion on asset retirement obligations	751	838	1,502	1,655
Loss on modification and extinguishment of debt		26,459		26,459
Asset impairment		0		0
Loss on partial settlement of benefit obligations	63		1,230
Cost impact of coal inventory fair value adjustment		0		0
Gain on assets acquired in an exchange transaction				0
Amortization of acquired intangibles, net	25	0	50	0
Adjusted EBITDA	(12,242)	$ (14,631)	(23,377)	$ (33,352)
All Other | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Asset impairment and restructuring	1,998		1,998
Restructuring expense	$ 1,993		1,993
All Other | Senior Management Changes
Segment Reporting, Other Significant Reconciling Item [Line Items]
Management restructuring costs			$ 435