The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Emergence from the Chapter 11 Cases and Fresh Start Reporting - Adjustments to PP&E (Details) - USD ($) $ in Millions	Sep. 30, 2017	Apr. 01, 2017	Dec. 31, 2016
Revaluation of Assets
Fresh-Start Adjustment [Line Items]
Fresh-start adjustment, increase (decrease), property, plant, equipment and mine development, net		$ (3,461.4)
Fresh-start adjustment, increase (decrease), accumulated depreciation, depletion and amortization		5,266.9
Predecessor
Fresh-Start Adjustment [Line Items]
Preconfirmation, property, plant, equipment and mine development, net		8,653.9
Preconfirmation, accumulated depreciation, depletion and amortization		(5,266.9)
Asset retirement obligations			$ 717.8
Successor
Fresh-Start Adjustment [Line Items]
Postconfirmation, property, plant, equipment and mine development, net		5,192.5
Postconfirmation, accumulated depreciation, depletion and amortization		0.0
Asset retirement obligations	$ 636.0
Land and coal interests \| Revaluation of Assets
Fresh-Start Adjustment [Line Items]
Fresh-start adjustment, increase (decrease), property, plant, equipment and mine development, net		(6,511.8)
Land and coal interests \| Predecessor
Fresh-Start Adjustment [Line Items]
Preconfirmation, property, plant, equipment and mine development, net		10,297.7
Land and coal interests \| Successor
Fresh-Start Adjustment [Line Items]
Postconfirmation, property, plant, equipment and mine development, net		3,785.9
PP&E, fair value		3,504.7
Asset retirement obligations		281.2
Buildings and improvements \| Revaluation of Assets
Fresh-Start Adjustment [Line Items]
Fresh-start adjustment, increase (decrease), property, plant, equipment and mine development, net		(1,013.2)
Buildings and improvements \| Predecessor
Fresh-Start Adjustment [Line Items]
Preconfirmation, property, plant, equipment and mine development, net		1,479.3
Buildings and improvements \| Successor
Fresh-Start Adjustment [Line Items]
Postconfirmation, property, plant, equipment and mine development, net		466.1
Machinery and equipment \| Revaluation of Assets
Fresh-Start Adjustment [Line Items]
Fresh-start adjustment, increase (decrease), property, plant, equipment and mine development, net		(1,203.3)
Machinery and equipment \| Predecessor
Fresh-Start Adjustment [Line Items]
Preconfirmation, property, plant, equipment and mine development, net		2,143.8
Machinery and equipment \| Successor
Fresh-Start Adjustment [Line Items]
Postconfirmation, property, plant, equipment and mine development, net		$ 940.5