Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended	12 Months Ended
	Apr. 01, 2017	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes				$ (223,200)	$ (390,900)
Asset impairment						$ 247,900	$ 1,277,800	$ 154,400
Income tax provision (benefit)	$ (266,000)	$ 4,700	$ (263,800)	(37,600)	(97,400)
DTA and PBGC
Reportable segment results
Net gain on disposal of assets			19,700
Successor
Reportable segment results
Revenues		1,258,300
Total Adjusted EBITDA		317,800
Gain on VEBA settlement		0
Net gain on disposal of assets		500
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes		101,400
Depreciation, depletion and amortization		148,300
Asset retirement obligation expenses		11,000
Selling and administrative expenses related to debt restructuring		0
Asset impairment		0
Change in deferred tax asset valuation allowance related to equity affiliates		(4,300)
Interest expense		41,400
Interest income		(1,500)
Break fees related to terminated asset sales		(28,000)
Realized losses on non-coal trading derivative contracts		0
Unrealized gains on non-coal trading derivative contracts		(3,200)
Unrealized (gains) losses on economic hedges		(9,400)
Coal inventory revaluation		67,300
Take-or-pay contract-based intangible recognition		(9,900)
Reorganization items, net		0
Income tax provision (benefit)		4,700
Total Adjusted EBITDA		317,800
Predecessor
Reportable segment results
Revenues	0		1,326,200	1,040,200	2,067,400
Total Adjusted EBITDA	0		341,300	72,600	107,800
Gain on VEBA settlement			0		68,100
Net gain on disposal of assets	0		22,800	13,700	15,500
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes	(319,800)		(195,500)	(223,200)	(390,900)
Depreciation, depletion and amortization	0		119,900	115,900	227,700
Asset retirement obligation expenses	0		14,600	11,500	24,600
Selling and administrative expenses related to debt restructuring	0		0	7,200	21,500
Asset impairment	0		30,500	0	17,200
Change in deferred tax asset valuation allowance related to equity affiliates	0		(5,200)	(1,400)	0
Interest expense	0		32,900	59,000	185,200
Interest income	0		(2,700)	(1,300)	(2,700)
Break fees related to terminated asset sales	0		0	0	0
Realized losses on non-coal trading derivative contracts	0		0	25,000	0
Unrealized gains on non-coal trading derivative contracts	0		0	0	0
Unrealized (gains) losses on economic hedges	0		(16,600)	22,100	27,200
Coal inventory revaluation	0		0	0	0
Take-or-pay contract-based intangible recognition	0		0	0	0
Reorganization items, net	585,800		627,200	95,400	95,400
Income tax provision (benefit)	(266,000)		(263,800)	(37,600)	(97,400)
Total Adjusted EBITDA	$ 0		341,300	72,600	107,800
Operating Segments | Successor | Powder River Basin Mining
Reportable segment results
Revenues		365,400
Total Adjusted EBITDA		84,800
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		84,800
Operating Segments | Successor | Midwestern U.S. Mining
Reportable segment results
Revenues		194,900
Total Adjusted EBITDA		46,500
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		46,500
Operating Segments | Successor | Western U.S. Mining
Reportable segment results
Revenues		125,400
Total Adjusted EBITDA		44,900
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		44,900
Operating Segments | Successor | Australian Metallurgical Mining
Reportable segment results
Revenues		287,800
Total Adjusted EBITDA		71,900
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		71,900
Operating Segments | Successor | Australian Thermal Mining
Reportable segment results
Revenues		239,200
Total Adjusted EBITDA		105,900
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		105,900
Operating Segments | Predecessor | Powder River Basin Mining
Reportable segment results
Revenues			394,300	306,600	642,600
Total Adjusted EBITDA			91,700	80,600	154,400
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			91,700	80,600	154,400
Operating Segments | Predecessor | Midwestern U.S. Mining
Reportable segment results
Revenues			193,200	189,000	388,600
Total Adjusted EBITDA			50,000	52,700	113,300
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			50,000	52,700	113,300
Operating Segments | Predecessor | Western U.S. Mining
Reportable segment results
Revenues			149,700	112,100	224,600
Total Adjusted EBITDA			50,000	28,800	48,900
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			50,000	28,800	48,900
Operating Segments | Predecessor | Australian Metallurgical Mining
Reportable segment results
Revenues			328,900	245,200	450,300
Total Adjusted EBITDA			109,600	(49,200)	(86,500)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			109,600	(49,200)	(86,500)
Operating Segments | Predecessor | Australian Thermal Mining
Reportable segment results
Revenues			224,800	186,800	363,500
Total Adjusted EBITDA			75,600	45,400	88,300
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			75,600	45,400	88,300
Corporate, Non-Segment | Successor | Trading and Brokerage
Reportable segment results
Revenues		5,200
Total Adjusted EBITDA		(5,100)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		(5,100)
Corporate, Non-Segment | Successor | Corporate and Other
Reportable segment results
Revenues		40,400
Total Adjusted EBITDA		(31,100)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		$ (31,100)
Corporate, Non-Segment | Predecessor | Trading and Brokerage
Reportable segment results
Revenues			15,000	17,500	13,800
Total Adjusted EBITDA			8,800	(18,200)	(31,900)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			8,800	(18,200)	(31,900)
Corporate, Non-Segment | Predecessor | Corporate and Other
Reportable segment results
Revenues			20,300	(17,000)	(16,000)
Total Adjusted EBITDA			(44,400)	(67,500)	(178,700)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			$ (44,400)	$ (67,500)	(178,700)
Funding of VEBA Pursuant to Settlement with UMWA and Patriot Coal Corporation | Spinoff | Patriot
Reportable segment results
Gain on VEBA settlement					$ 68,100