Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,385.9)	$ (1,321.3)
Property, plant, equipment and mine development, net	3,025.4	3,051.1
Land and coal interests
Property, Plant and Equipment [Line Items]
Land and coal interests	2,484.1	2,482.9
Buildings and improvements
Property, Plant and Equipment [Line Items]
Land and coal interests	484.7	481.0
Machinery and equipment
Property, Plant and Equipment [Line Items]
Land and coal interests	$ 1,442.5	$ 1,408.5

Amount, before accumulated depreciation and amortization, of property, plant, and equipment and finance lease right-of-use asset.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(13))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123391704&loc=SL77918627-209977

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