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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

Name:	us-gaap_AssetRetirementObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of fresh-start adjustment to accumulated depreciation and amortization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6918054&loc=SL2890621-112765

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

+ Details

Name:	us-gaap_FreshStartAdjustmentIncreaseDecreaseAccumulatedDepreciationAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total fresh-start adjustment to property and equipment, net.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6918054&loc=SL2890621-112765

+ Details

Name:	us-gaap_FreshStartAdjustmentIncreaseDecreasePropertyAndEquipmentNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FreshStartAdjustmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accumulated depreciation and amortization at the balance sheet date after fresh-start adjustments.

+ References

+ Details

Name:	us-gaap_PostconfirmationAccumulatedDepreciationAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total property and equipment, net at the balance sheet date after fresh-start adjustments.

+ References

+ Details

Name:	us-gaap_PostconfirmationPropertyAndEquipmentNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated depreciation and amortization at the balance sheet date before fresh-start adjustments.

+ References

+ Details

Name:	us-gaap_PreconfirmationAccumulatedDepreciationAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total property and equipment, net at the balance sheet date before fresh-start adjustments.

+ References

+ Details

Name:	us-gaap_PreconfirmationPropertyAndEquipmentNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FreshStartAdjustmentsTypeOfFreshStartAdjustmentAxis=us-gaap_RevaluationOfAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_PredecessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_SuccessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ProductiveLandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingAndBuildingImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_MachineryAndEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Adjustments to PP&E (Details) - USD ($) $ in Millions	Dec. 31, 2017	Apr. 01, 2017	Dec. 31, 2016	Dec. 31, 2015
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		664.2	$ 758.8	$ 712.1
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	$ 691.1	664.2
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 [Member] \| 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 [Member] \| Predecessor
Fresh-Start Adjustment [Line Items]
Preconfirmation, property, plant, equipment and mine development, net		1,479.3
Buildings and improvements [Member] \| 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