Segment Information - Reconciliation of Net Income (Loss) to Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	$ (39,808)	$ 7,990
Interest expense	17,605	15,155
Interest income	(978)	(1,936)
Income tax benefit	(2,188)	(4,778)
Depreciation, depletion and amortization	54,465	61,271
Merger-related costs	0	831
Non-cash stock compensation expense	2,078	5,271
Mark-to-market adjustment for acquisition-related obligations	(14,997)	1,936
Accretion on asset retirement obligations	7,375	6,232
Asset impairment	33,709	0
Management restructuring costs	947
Loss on partial settlement of benefit obligations	1,167
Cost impact of coal inventory fair value adjustment		7,176
Gain on assets acquired in an exchange transaction	0	(9,083)
Amortization of acquired intangibles, net	865	(6,683)
Adjusted EBITDA	60,240	83,382
Operating Segments | CAPP - Met
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(11,464)	70,384
Interest expense	(930)	28
Interest income	(58)	(4)
Income tax benefit	0	0
Depreciation, depletion and amortization	41,722	36,673
Merger-related costs		0
Non-cash stock compensation expense	399	403
Mark-to-market adjustment for acquisition-related obligations	0	0
Accretion on asset retirement obligations	3,502	2,333
Asset impairment	32,951
Management restructuring costs	501
Loss on partial settlement of benefit obligations	0
Cost impact of coal inventory fair value adjustment		3,718
Gain on assets acquired in an exchange transaction		(9,083)
Amortization of acquired intangibles, net	2,581	(2,820)
Adjusted EBITDA	69,204	101,632
Operating Segments | CAPP - Thermal
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(8,066)	(19,405)
Interest expense	2	4
Interest income	0	0
Income tax benefit	0	0
Depreciation, depletion and amortization	4,849	14,112
Merger-related costs		0
Non-cash stock compensation expense	7	52
Mark-to-market adjustment for acquisition-related obligations	0	0
Accretion on asset retirement obligations	2,352	2,065
Asset impairment	758
Management restructuring costs	5
Loss on partial settlement of benefit obligations	0
Cost impact of coal inventory fair value adjustment		3,458
Gain on assets acquired in an exchange transaction		0
Amortization of acquired intangibles, net	(2,095)	(4,569)
Adjusted EBITDA	(2,188)	(4,283)
Operating Segments | NAPP
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(3,040)	(3,585)
Interest expense	(570)	1
Interest income	(10)	(12)
Income tax benefit	0	0
Depreciation, depletion and amortization	6,849	6,627
Merger-related costs		0
Non-cash stock compensation expense	0	0
Mark-to-market adjustment for acquisition-related obligations	0	0
Accretion on asset retirement obligations	770	1,017
Asset impairment	0
Management restructuring costs	6
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	354	706
Adjusted EBITDA	4,359	4,754
All Other
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net (loss) income from continuing operations	(17,238)	(39,404)
Interest expense	19,103	15,122
Interest income	(910)	(1,920)
Income tax benefit	(2,188)	(4,778)
Depreciation, depletion and amortization	1,045	3,859
Merger-related costs		831
Non-cash stock compensation expense	1,672	4,816
Mark-to-market adjustment for acquisition-related obligations	(14,997)	1,936
Accretion on asset retirement obligations	751	817
Asset impairment	0
Management restructuring costs	435
Loss on partial settlement of benefit obligations	1,167
Cost impact of coal inventory fair value adjustment		0
Gain on assets acquired in an exchange transaction		0
Amortization of acquired intangibles, net	25	0
Adjusted EBITDA	$ (11,135)	$ (18,721)