Amount of net gain (loss) on settlement of other claims for entities in bankruptcy, reported as a reorganization item.

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Emergence from Bankruptcy and Fresh Start Accounting (Reorganization Adjustments and Property, Plant and Equipment) (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
	Oct. 01, 2016	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2015
Fresh-Start Adjustment [Line Items]
Total pre-tax gain on plan effects	$ 4,142,104
Property, plant and equipment, net		$ 1,053,603		$ 955,948
Fresh-Start Adjustment, Increase (Decrease), Property and Equipment, Net	(2,363,829)		$ (2,363,829)
Net Coal Properties
Fresh-Start Adjustment [Line Items]
Fresh-Start Adjustment, Increase (Decrease), Property and Equipment, Net	(1,971,314)		(1,971,314)
Net Plant & Equipment
Fresh-Start Adjustment [Line Items]
Fresh-Start Adjustment, Increase (Decrease), Property and Equipment, Net	(405,259)		(405,259)
Net Deferred Charges
Fresh-Start Adjustment [Line Items]
Fresh-Start Adjustment, Increase (Decrease), Property and Equipment, Net	12,744		12,744
Predecessor
Fresh-Start Adjustment [Line Items]
Total pre-tax gain on plan effects	4,142,104		4,142,104		$ 0
Cancellation of predecessor common stock	2,145		2,145
Cancellation of predecessor paid-in capital	3,056,307		3,056,307
Cancellation of predecessor treasury stock	(53,863)		(53,863)
Net impact on accumulated earnings (deficit)	7,146,693		7,146,693
Property, plant and equipment, net	3,434,941		3,434,941
Predecessor \| Net Coal Properties
Fresh-Start Adjustment [Line Items]
Property, plant and equipment, net	2,358,779		2,358,779
Predecessor \| Net Plant & Equipment
Fresh-Start Adjustment [Line Items]
Property, plant and equipment, net	812,888		812,888
Predecessor \| Net Deferred Charges
Fresh-Start Adjustment [Line Items]
Property, plant and equipment, net	263,274		263,274
Successor
Fresh-Start Adjustment [Line Items]
Total pre-tax gain on plan effects		$ 0		$ 0
Property, plant and equipment, net	1,071,112		1,071,112
Successor \| Net Coal Properties
Fresh-Start Adjustment [Line Items]
Property, plant and equipment, net	387,465		387,465
Successor \| Net Plant & Equipment
Fresh-Start Adjustment [Line Items]
Property, plant and equipment, net	407,629		407,629
Successor \| Net Deferred Charges
Fresh-Start Adjustment [Line Items]
Property, plant and equipment, net	$ 276,018		$ 276,018