Amount classified as assets attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer.

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

Fixed Assets and Mineral Leaseholds (Disposal Groups, Including Discontinued Operations) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Aug. 07, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	$ 255,519		$ 0
Non-Core Assets Powder River Basin [Member] \| Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Option payment on assets held for sale		$ 30,000
Assets held for sale	$ 255,500