Asset Impairment	12 Months Ended
Dec. 31, 2018
Asset Impairment and Mine Closure Costs [Abstract]
Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block]	Asset Impairment The Company’s mining and exploration assets and mining-related investments may be adversely affected by numerous uncertain factors that may cause the Company to be unable to recover all or a portion of the carrying value of those assets. The Company generally does not view short-term declines in thermal and metallurgical coal prices as an indicator of impairment. However, the Company generally views a sustained trend (for example, over periods exceeding one year) of adverse coal pricing or unfavorable changes thereto as a potential indicator of impairment. Because of the volatile and cyclical nature of coal prices and demand, it is reasonably possible that coal prices may decrease and/or fail to improve in the near term, which, absent sufficient mitigation such as an offsetting reduction in the Company’s operating costs, may result in the need for future adjustments to the carrying value of the Company’s long-lived mining assets and mining-related investments. The Company's assets whose recoverability and values are most sensitive to near-term pricing include certain Australian metallurgical assets with an aggregate carrying value of $177.2 million as of December 31, 2018. The Company conducted a review of those assets for recoverability as of December 31, 2018 and determined that no further impairment charge was necessary as of that date. The fair value estimates made during the Company’s impairment assessments were determined in accordance with the methods outlined in Note 1. “Summary of Significant Accounting Policies,” except in certain instances where indicative bids were received related to non-strategic assets being marketed for divestiture. In those instances, the indicative bids were also considered in estimating fair value. As described in Note 2. “Emergence from the Chapter 11 Cases and Fresh Start Reporting,” the Company adjusted the book values of its property, plant, equipment and mine development assets to estimated fair value in connection with fresh start reporting. During the year ended December 31, 2018 and the period April 2 through December 31, 2017, the Company recognized no impairment charges. During the period January 1 through April 1, 2017, the Company recognized impairment charges of $30.5 million related to terminated coal lease contracts in the Midwestern United States. During 2016, the Company recorded an impairment charge of $193.2 million related to its Metropolitan Mine in New South Wales, Australia as a result of entering into a definitive agreement to sell the mine for an amount less than the carrying value of the related assets. The agreement was subsequently terminated during 2017 as the parties were unable to obtain the necessary Australian regulatory approvals, but the agreed-upon selling price was deemed the best estimate of fair value for the related assets. Also during 2016, during a review of its asset portfolio and prepetition leases, the Company identified certain non-strategic Midwestern coal reserves held under lease that were determined to be uneconomical to be mined in the future. As a result, the Company rejected certain leases and recognized an aggregate impairment charge of $37.5 million. The Company also recognized a $17.2 million impairment charge to record at fair value certain non-strategic Australian metallurgical assets classified as held for sale. For additional information regarding divested assets, refer to Note 22. “Other Events.”

Asset Impairment

The Company’s mining and exploration assets and mining-related investments may be adversely affected by numerous uncertain factors that may cause the Company to be unable to recover all or a portion of the carrying value of those assets. The Company generally does not view short-term declines in thermal and metallurgical coal prices as an indicator of impairment. However, the Company generally views a sustained trend (for example, over periods exceeding one year) of adverse coal pricing or unfavorable changes thereto as a potential indicator of impairment. Because of the volatile and cyclical nature of coal prices and demand, it is reasonably possible that coal prices may decrease and/or fail to improve in the near term, which, absent sufficient mitigation such as an offsetting reduction in the Company’s operating costs, may result in the need for future adjustments to the carrying value of the Company’s long-lived mining assets and mining-related investments.

The Company's assets whose recoverability and values are most sensitive to near-term pricing include certain Australian metallurgical assets with an aggregate carrying value of $177.2 million as of December 31, 2018. The Company conducted a review of those assets for recoverability as of December 31, 2018 and determined that no further impairment charge was necessary as of that date.

The fair value estimates made during the Company’s impairment assessments were determined in accordance with the methods outlined in Note 1. “Summary of Significant Accounting Policies,” except in certain instances where indicative bids were received related to non-strategic assets being marketed for divestiture. In those instances, the indicative bids were also considered in estimating fair value. As described in Note 2. “Emergence from the Chapter 11 Cases and Fresh Start Reporting,” the Company adjusted the book values of its property, plant, equipment and mine development assets to estimated fair value in connection with fresh start reporting.

During the year ended December 31, 2018 and the period April 2 through December 31, 2017, the Company recognized no impairment charges. During the period January 1 through April 1, 2017, the Company recognized impairment charges of $30.5 million related to terminated coal lease contracts in the Midwestern United States.

During 2016, the Company recorded an impairment charge of $193.2 million related to its Metropolitan Mine in New South Wales, Australia as a result of entering into a definitive agreement to sell the mine for an amount less than the carrying value of the related assets. The agreement was subsequently terminated during 2017 as the parties were unable to obtain the necessary Australian regulatory approvals, but the agreed-upon selling price was deemed the best estimate of fair value for the related assets. Also during 2016, during a review of its asset portfolio and prepetition leases, the Company identified certain non-strategic Midwestern coal reserves held under lease that were determined to be uneconomical to be mined in the future. As a result, the Company rejected certain leases and recognized an aggregate impairment charge of $37.5 million. The Company also recognized a $17.2 million impairment charge to record at fair value certain non-strategic Australian metallurgical assets classified as held for sale. For additional information regarding divested assets, refer to Note 22. “Other Events.”