Segment Information (Reconciliation of Segment Income from Operations to Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands			3 Months Ended											6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 07, 2017	Oct. 01, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2015	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2015
Segment Reporting Information [Line Items]
Income (loss) before income taxes							$ 34,605									$ 203,195
Interest expense, net							10,754									24,256
Depreciation, depletion and amortization							32,604									122,464
Accretion on asset retirement obligations							7,634									30,209
Amortization of sales contracts, net							796									53,985
Asset impairment and mine closure costs							0				$ 500,000	$ 2,100,000	$ 19,100	$ 2,600,000		0
Losses from disposed operations resulting from Patriot Coal bankruptcy							0									0
Gain on sale of Lone Mountain Processing, Inc.							0									(21,297)
Net loss resulting from early retirement of debt and debt restructuring	$ 0						0									2,547
Successor
Segment Reporting Information [Line Items]
Income (loss) before income taxes							34,605									203,195
Interest expense, net							10,754									24,256
Depreciation, depletion and amortization							32,604									122,464
Accretion on asset retirement obligations							7,634									30,209
Amortization of sales contracts, net							796									53,985
Asset impairment and mine closure costs							0									0
Losses from disposed operations resulting from Patriot Coal bankruptcy							0									0
Gain on sale of Lone Mountain Processing, Inc.							0									(21,297)
Net loss resulting from early retirement of debt and debt restructuring							0									2,547
Reorganization items, net			$ (494)				759									2,398
Fresh start coal inventory fair value adjustment							7,345									0
Adjusted EBITDAR							94,497									$ 417,757
Predecessor
Segment Reporting Information [Line Items]
Income (loss) before income taxes															$ 1,237,455		$ (3,286,522)
Interest expense, net															133,235		393,549
Depreciation, depletion and amortization															191,581		379,345
Accretion on asset retirement obligations															24,321		33,680
Amortization of sales contracts, net															(728)		(8,811)
Asset impairment and mine closure costs								$ 46	$ 43,701	$ 85,520					129,267		2,628,303
Losses from disposed operations resulting from Patriot Coal bankruptcy															0		116,343
Gain on sale of Lone Mountain Processing, Inc.															0		0
Net loss resulting from early retirement of debt and debt restructuring															2,213		27,910
Reorganization items, net		$ (1,676,091)		$ 43	$ 21	$ 2,828	$ 759	$ 20,904	$ 21,271	$ 3,875					(1,630,041)		0
Fresh start coal inventory fair value adjustment															0		0
Adjusted EBITDAR															$ 87,303		$ 283,797