Segment Information - Schedule of Operating Results and Capital Expenditures (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) mine plant segment	Jun. 30, 2019 USD ($) segment
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3	4
Total revenues	$ 411,838	$ 656,206	$ 882,298	$ 1,265,320
Depreciation, depletion and amortization	49,262	62,814	103,727	124,085
Amortization of acquired intangibles, net	2,096	(343)	2,961	(7,026)
Adjusted EBITDA	16,903	140,752	77,143	224,134
Capital expenditures	41,531	42,798	$ 91,090	83,882
CAPP - Met | Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine			4
Number of preparation plants | plant			2
CAPP - Met | West Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine			18
Number of preparation plants | plant			5
CAPP - Thermal | West Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine			3
Number of preparation plants | plant			2
NAPP | Pennsylvania
Segment Reporting Information [Line Items]
Number of active mines | mine			1
Number of preparation plants | plant			1
Number of closed mines | mine			1
Operating Segments | CAPP - Met
Segment Reporting Information [Line Items]
Total revenues	316,534	494,525	$ 679,293	967,328
Depreciation, depletion and amortization	38,800	38,829	80,522	75,502
Amortization of acquired intangibles, net	2,759	3,870	5,340	1,050
Adjusted EBITDA	17,617	123,051	86,821	224,683
Capital expenditures	27,647	28,106	60,781	57,692
Operating Segments | CAPP - Thermal
Segment Reporting Information [Line Items]
Total revenues	36,313	82,035	75,581	145,266
Depreciation, depletion and amortization	7,260	16,502	12,109	30,614
Amortization of acquired intangibles, net	(903)	(4,213)	(2,998)	(8,782)
Adjusted EBITDA	2,492	11,034	304	6,751
Capital expenditures	2,504	5,190	5,584	7,659
Operating Segments | NAPP
Segment Reporting Information [Line Items]
Total revenues	57,986	78,928	125,543	151,304
Depreciation, depletion and amortization	2,172	6,522	9,021	13,149
Amortization of acquired intangibles, net	215	0	569	706
Adjusted EBITDA	9,036	21,298	13,395	26,052
Capital expenditures	10,873	8,204	24,169	16,203
All Other
Segment Reporting Information [Line Items]
Total revenues	1,005	718	1,881	1,422
Depreciation, depletion and amortization	1,030	961	2,075	4,820
Amortization of acquired intangibles, net	25	0	50	0
Adjusted EBITDA	(12,242)	(14,631)	(23,377)	(33,352)
Capital expenditures	$ 507	$ 1,298	$ 556	$ 2,328