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Derivatives and Fair Value Measurements (Policies) | 9 Months Ended
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Sep. 30, 2018
Fair Value Disclosures [Abstract] |
Corporate Hedging [Policy Text Block] | The Company is exposed to several risks in the normal course of business, including (1) foreign currency exchange rate risk for non-U.S. dollar expenditures and balances, (2) price risk on coal produced by and diesel fuel utilized in the Company’s mining operations and (3) interest rate risk that has been partially mitigated by fixed rates on long-term debt. The Company manages a portion of its price risk related to the sale of coal (excluding coal trading activities) using long-term coal supply agreements. Derivative financial instruments have historically been used to manage the Company’s exposure to foreign currency exchange rate risk, primarily on Australian dollar expenditures made in its Australian mining platform. This risk was historically managed using forward contracts and options designated as cash flow hedges, with the objective of reducing the variability of cash flows associated with forecasted foreign currency expenditures. The Company previously used derivative instruments to manage its exposure to the variability of diesel fuel prices used in production in the U.S. and Australia with swaps or options, which it also designated as cash flow hedges, with the objective of reducing the variability of cash flows associated with forecasted diesel fuel purchases. These risk management activities are collectively referred to as “Corporate Hedging” and are actively monitored for compliance with the Company’s risk management policies.
Fair Value Transfer, Derivatives [Policy Text Block] | The Company’s policy is to value all transfers between levels using the beginning of period valuation. The Company’s policy is to value all transfers between levels using the beginning of period valuation.

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- Definition Corporate Hedging [Policy Text Block] + References No definition available. + Details
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- Definition Disclosure of accounting policy for determining when transfers between levels are recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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