Segment Information - Reconciliation of Net Income (Loss) (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017
Successor
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 191,636,000	$ (18,451,000)		$ 326,548,000
Other revenues	11,582,000	6,059,000		24,490,000
Cost of other revenues	(7,795,000)	(5,126,000)		(15,974,000)
Depreciation and depletion	(19,650,000)	(15,821,000)		(34,232,000)
Selling, general and administrative	(8,660,000)	(5,815,000)		(13,830,000)
Other postretirement benefits	0	0		0
Restructuring charges	0	0		0
Transaction and other costs	(3,837,000)	(10,475,000)		(12,873,000)
Interest expense, net	(642,000)	(434,000)		(1,250,000)
Reorganization items, net	0	0		0
Income tax expense	(32,769,000)	0		(34,706,000)
Net income (loss)	129,865,000	$ (50,063,000)		$ 238,173,000
Predecessor
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA			$ (7,143,000)
Other revenues			6,229,000
Cost of other revenues			(4,698,000)
Depreciation and depletion			(28,958,000)
Selling, general and administrative			(9,008,000)
Other postretirement benefits			(6,160,000)
Restructuring charges			(3,418,000)
Transaction and other costs			0
Interest expense, net			(16,562,000)
Reorganization items, net	$ 7,920,000		7,920,000
Income tax expense			(18,000)
Net income (loss)			$ (61,816,000)