Subsequent Events (Details) - High-Vol A cooking coal reserve, adjacent to Leer Mine T in Millions, $ in Millions	1 Months Ended
Sep. 30, 2019 USD ($) T
Subsequent Event [Line Items]
Reserve, mass (in tons) \| T	20
Payments to acquire productive assets \| $	$ 52.5
Productive asset, increase to useful life	6 years

The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585

Name:	us-gaap_PaymentsToAcquireProductiveAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration