Discontinued Operations - Major Components of Cash Flows (Details) - PRB Transaction - Discontinued Operations - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciation, depletion and amortization	$ 0	$ 145,913
Accretion on asset retirement obligations	4,981	4,981
Asset impairment	$ 694	$ 17,162

Amount, before tax, of (gain) loss recognized for the (reversal of write-down) write-down to fair value, less cost to sell, of a disposal group. Excludes discontinued operations.

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