Segment Information - Schedule of Operating Results and Capital Expenditures (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment plant mine	Sep. 30, 2019 segment	Mar. 31, 2019 USD ($) mine
Segment Reporting Information [Line Items]
Number of reportable segments | segment	3	4
Total revenues	$ 470,460		$ 609,114
Depreciation, depletion and amortization	54,465		61,271
Amortization of acquired intangibles, net	865		(6,683)
Adjusted EBITDA	60,240		83,382
Capital expenditures	$ 49,559		$ 41,084
CAPP - Met | Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine	5
Number of preparation plants | plant	2
CAPP - Met | West Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine			16
Number of preparation plants | plant	5
CAPP - Thermal | West Virginia
Segment Reporting Information [Line Items]
Number of active mines | mine	4
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	$ 362,759		$ 472,803
Depreciation, depletion and amortization	41,722		36,673
Amortization of acquired intangibles, net	2,581		(2,820)
Adjusted EBITDA	69,204		101,632
Capital expenditures	33,134		29,586
Operating Segments | CAPP - Thermal
Segment Reporting Information [Line Items]
Total revenues	39,268		63,231
Depreciation, depletion and amortization	4,849		14,112
Amortization of acquired intangibles, net	(2,095)		(4,569)
Adjusted EBITDA	(2,188)		(4,283)
Capital expenditures	3,080		2,469
Operating Segments | NAPP
Segment Reporting Information [Line Items]
Total revenues	67,557		72,376
Depreciation, depletion and amortization	6,849		6,627
Amortization of acquired intangibles, net	354		706
Adjusted EBITDA	4,359		4,754
Capital expenditures	13,296		7,999
All Other
Segment Reporting Information [Line Items]
Total revenues	876		704
Depreciation, depletion and amortization	1,045		3,859
Amortization of acquired intangibles, net	25		0
Adjusted EBITDA	(11,135)		(18,721)
Capital expenditures	$ 49		$ 1,030