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Asset Impairment	12 Months Ended
Asset Impairment	Dec. 31, 2019
Asset Impairment and Mine Closure Costs [Abstract]
Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block]	Asset Impairment During the year ended December 31, 2019, the Company recognized impairment charges of $172.0 million related to its El Segundo/Lee Ranch Mine of the Western U.S. Mining segment and $20.0 million related to its Wildcat Hills Underground Mine of the Midwestern U.S. Mining segment based upon the expectation of reduced sales volumes and uncertainty over remaining economic mine lives. The related impairment charges were based upon the remaining probability-weighted discounted cash flows of those mines. The Company also recognized impairment charges of $69.2 million related to certain unallocated coal reserves in the Midwest and Colorado due to their low probability of development, and $9.0 million related to the fair value of an investment in equity securities during the year ended December 31, 2019. 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 Midwest. In addition to the impairment charges described above, the Company also recorded provisions related to its North Goonyella Mine during the year ended December 31, 2019, as further described at Note 22. “Other Events.” In connection with its impairment assessments, the Company identified long-lived assets included in its Midwestern U.S. Mining and Corporate and Other segments whose recoverability and carrying values were most sensitive to coal pricing, cost pressures and customer concentration risk at December 31, 2019. Such assets had an aggregate carrying value of $89.0 million as of December 31, 2019. The Company conducted a review of those assets for recoverability and determined that no further impairment charge was necessary as of that date.