Segment Information - Schedule of Operating Results and Capital Expenditures (Details) $ in Thousands		3 Months Ended								6 Months Ended	12 Months Ended
	Nov. 08, 2018 segment	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 segments	Dec. 31, 2019 USD ($) segment plant mine	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments	3									4	3
Total revenues		$ 499,076	$ 525,864	$ 656,206	$ 609,114	$ 572,084	$ 447,871	$ 528,918	$ 482,332		$ 2,290,260	$ 2,031,205	$ 1,649,969
Depreciation, depletion and amortization											228,792	77,549	34,910
Amortization of acquired intangibles, net											(88)	(5,392)	59,007
Adjusted EBITDA											295,607	335,076	278,466
Capital expenditures											192,411	81,881	72,701
All Other
Segment Reporting Information [Line Items]
Total revenues											3,534	3,297	835
Depreciation, depletion and amortization											5,439	3,350	882
Amortization of acquired intangibles, net											0	0	0
Adjusted EBITDA											(63,883)	(43,552)	(40,281)
Capital expenditures											2,652	332	1,200
Met | Operating Segments
Segment Reporting Information [Line Items]
Total revenues											1,711,252	1,671,219	1,329,734
Depreciation, depletion and amortization											153,006	40,330	18,941
Amortization of acquired intangibles, net											10,389	(12,334)	34,737
Adjusted EBITDA											316,324	335,135	264,314
Capital expenditures											$ 140,250	39,634	20,494
Met | Virgina
Segment Reporting Information [Line Items]
Number of active mines | mine											7
Number of preparation plants | plant											2
Met | West Virgina
Segment Reporting Information [Line Items]
Number of active mines | mine											16
Number of preparation plants | plant											5
Thermal | Operating Segments
Segment Reporting Information [Line Items]
Total revenues											$ 286,486	39,650	0
Depreciation, depletion and amortization											57,483	10,596	0
Amortization of acquired intangibles, net											(13,578)	(7,516)	0
Adjusted EBITDA											11,981	(875)	0
Capital expenditures											$ 17,545	1,280	0
Thermal | West Virgina
Segment Reporting Information [Line Items]
Number of active mines | mine											5
Number of preparation plants | plant											2
NAPP | Operating Segments
Segment Reporting Information [Line Items]
Total revenues											$ 288,988	317,039	319,400
Depreciation, depletion and amortization											12,864	23,273	15,087
Amortization of acquired intangibles, net											3,101	14,458	24,270
Adjusted EBITDA											31,185	44,368	54,433
Capital expenditures											$ 31,964	$ 40,635	$ 51,007
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