Segment Information - Schedule of Operating Results and Capital Expenditures (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) plant mine segment	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Total revenues	$ 400,691	$ 525,864	$ 1,282,989	$ 1,791,184
Depreciation, depletion and amortization	50,739	60,842	154,466	184,927
Amortization of acquired intangibles, net	2,219	2,314	5,180	(4,712)
Adjusted EBITDA	19,687	40,023	96,830	264,157
Capital expenditures	27,806	60,301	$ 118,896	144,183
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			17
Number of preparation plants | plant			5
CAPP - Thermal | West Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine			1
Number of preparation plants | plant			1
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	295,381	373,356	$ 974,674	1,340,684
Depreciation, depletion and amortization	41,177	38,212	121,699	113,714
Amortization of acquired intangibles, net	2,535	4,765	7,875	5,816
Adjusted EBITDA	17,847	58,796	104,668	283,483
Capital expenditures	22,668	47,316	83,449	105,008
Operating Segments | CAPP - Thermal
Segment Reporting Information [Line Items]
Total revenues	39,835	80,432	115,416	225,698
Depreciation, depletion and amortization	7,313	13,972	19,422	44,586
Amortization of acquired intangibles, net	(486)	(3,359)	(3,484)	(12,142)
Adjusted EBITDA	5,155	1,954	5,459	8,704
Capital expenditures	765	5,706	6,349	13,365
Operating Segments | NAPP
Segment Reporting Information [Line Items]
Total revenues	65,100	71,446	190,643	222,750
Depreciation, depletion and amortization	1,504	6,241	10,525	19,390
Amortization of acquired intangibles, net	145	908	714	1,614
Adjusted EBITDA	7,250	(4,152)	20,645	21,900
Capital expenditures	4,299	7,114	28,468	23,317
All Other
Segment Reporting Information [Line Items]
Total revenues	375	630	2,256	2,052
Depreciation, depletion and amortization	745	2,417	2,820	7,237
Amortization of acquired intangibles, net	25	0	75	0
Adjusted EBITDA	(10,565)	(16,575)	(33,942)	(49,930)
Capital expenditures	$ 74	$ 165	$ 630	$ 2,493