Basis of Presentation	12 Months Ended
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include the accounts of Arch Coal, Inc. and its subsidiaries and controlled entities (the “Company”). The Company’s primary business is the production of thermal and metallurgical coal from surface and underground mines located throughout the United States, for sale to utility, industrial and steel producers both in the United States and around the world. The Company currently operates mining complexes in West Virginia, Kentucky, Maryland, Virginia, Illinois, Wyoming and Colorado. All subsidiaries are wholly-owned. Intercompany transactions and accounts have been eliminated in consolidation. Filing Under Chapter 11 of the United States Bankruptcy Code On January 11, 2016 (the “Petition Date”), Arch and substantially all of its wholly owned domestic subsidiaries (the “Filing Subsidiaries” and, together with Arch, the “Debtors”) filed voluntary petitions for reorganization (collectively, the “Bankruptcy Petitions”) under Chapter 11 of Title 11 of the U.S. Code (the “Bankruptcy Code”) in the United States Bankruptcy Court for the Eastern District of Missouri (the “Court”). The Debtor’s Chapter 11 Cases (collectively, the “Chapter 11 Cases”) are being jointly administered under the caption In re Arch Coal, Inc., et al. Case No. 16-40120 (lead case). Each Debtor will continue to operate its business as a “debtor in possession” under the jurisdiction of the Court and in accordance with the applicable provisions of the Bankruptcy Code and the orders of the Court. The filing of the Bankruptcy Petitions constituted an event of default that accelerated Arch’s obligations under the documents governing each of Arch’s 7.00% senior notes due 2019, 9.875% senior notes due 2019, 8.00% senior secured second lien notes due 2019, 7.25% senior notes due 2020, 7.25% senior notes due 2021 (together, the “senior notes”) and senior secured first lien term loan due 2018 (the “Existing Credit Agreement”) (collectively with the senior notes, the “Debt Instruments”), all as further described in Note 14, "Debt and Financing Arrangements" to the Consolidated Financial Statements included in the Form 10-K. Immediately after filing the Bankruptcy Petitions, Arch began notifying all known current or potential creditors of the Debtors of the bankruptcy filings. Additionally, on the Petition Date, the New York Stock Exchange (the “NYSE”) determined that the Company's stock was no longer suitable for listing pursuant to Section 8.02.01D of the NYSE continued listing standards and trading in the Company’s common stock was suspended on January 11, 2016. We expect that the existing common stock of the Company will be extinguished upon the Company's emergence from Chapter 11 and existing equity holders will not receive consideration in respect of their equity interests. On the Petition Date, the Debtors filed a number of motions with the Court generally designed to stabilize their operations and facilitate the Debtors’ transition into Chapter 11. Certain of these motions sought authority from the Court for the Debtors to make payments upon, or otherwise honor, certain pre-petition obligations (e.g., obligations related to certain employee wages, salaries and benefits and certain vendors and other providers essential to the Debtors’ businesses). The Court has entered orders approving the relief sought in these motions, in certain cases on an interim basis. Pursuant to Section 362 of the Bankruptcy Code, the filing of the Bankruptcy Petitions automatically stayed most actions against the Debtors, including actions to collect indebtedness incurred prior to the Petition Date or to exercise control over the Debtors’ property. Subject to certain exceptions under the Bankruptcy Code, the filing of the Debtors’ Chapter 11 Cases also automatically stayed the continuation of most legal proceedings, including the third party litigation matters described under “Legal Proceedings,” or the filing of other actions against or on behalf of the Debtors or their property to recover on, collect or secure a claim arising prior to the Petition Date or to exercise control over property of the Debtors’ bankruptcy estates, unless and until the Court modifies or lifts the automatic stay as to any such claim. Notwithstanding the general application of the automatic stay described above, governmental authorities may determine to continue actions brought under their police and regulatory powers. As required by the Bankruptcy Code, the U.S. Trustee for the Eastern District of Missouri appointed an official committee of unsecured creditors (the “Creditors’ Committee”) on January 25, 2016. The Creditors’ Committee represents all unsecured creditors of the Debtors and has a right to be heard on all matters that come before the Court. As a result of extremely challenging current market conditions, the Company believes it will require a significant restructuring of its balance sheet in order to continue as a going concern in the long term. The Company’s ability to continue as a going concern is dependent upon, among other things, improvement in current market conditions, its ability to become profitable and maintain profitability and its ability to successfully implement its Chapter 11 plan strategy. As a result of the Bankruptcy Petitions, the realization of the Debtors’ assets and the satisfaction of liabilities are subject to significant uncertainty. While operating as a debtor-in-possession pursuant to the Bankruptcy Code, the Company may sell or otherwise dispose of or liquidate assets or settle liabilities, subject to the approval of the Court or as otherwise permitted in the ordinary course of business for amounts other than those reflected in the accompanying consolidated financial statements. Further, a Chapter 11 plan is likely to materially change the amounts and classifications of assets and liabilities reported in the Company’s Consolidated Financial Statements. On August 4, 2015, the Company effected a 1-for-10 reverse stock split of its common stock. Each stockholder's percentage ownership and proportional voting power remain unchanged as a result of the reverse stock split. All applicable share data, per share amounts and related information in the Consolidated Financial Statements and notes thereto have been adjusted retroactively to give effect to the 1-for-10 reverse stock split. The Company completed the sale of Canyon Fuel Company, LLC (Canyon Fuel) on August 16, 2013. The results of these mining complexes have been segregated from continuing operations and are reflected, net of tax, as discontinued operations in the consolidated statements of operations for 2013. See further discussion in Note 3, "Divestitures". In response to weak coal markets, the Company has idled or closed mines in the Appalachia region and sold other non-core operating subsidiaries and assets. The results from these operations and gains or losses on the disposal are reflected in income from continuing operations in the consolidated statements of operations. See further discussion in Note 5, "Impairment Charges and Mine Closure Costs".

Basis of Presentation

The accompanying consolidated financial statements include the accounts of Arch Coal, Inc. and its subsidiaries and controlled entities (the “Company”). The Company’s primary business is the production of thermal and metallurgical coal from surface and underground mines located throughout the United States, for sale to utility, industrial and steel producers both in the United States and around the world. The Company currently operates mining complexes in West Virginia, Kentucky, Maryland, Virginia, Illinois, Wyoming and Colorado. All subsidiaries are wholly-owned. Intercompany transactions and accounts have been eliminated in consolidation.

Filing Under Chapter 11 of the United States Bankruptcy Code

On January 11, 2016 (the “Petition Date”), Arch and substantially all of its wholly owned domestic subsidiaries (the “Filing Subsidiaries” and, together with Arch, the “Debtors”) filed voluntary petitions for reorganization (collectively, the “Bankruptcy Petitions”) under Chapter 11 of Title 11 of the U.S. Code (the “Bankruptcy Code”) in the United States Bankruptcy Court for the Eastern District of Missouri (the “Court”). The Debtor’s Chapter 11 Cases (collectively, the “Chapter 11 Cases”) are being jointly administered under the caption In re Arch Coal, Inc., et al. Case No. 16-40120 (lead case). Each Debtor will continue to operate its business as a “debtor in possession” under the jurisdiction of the Court and in accordance with the applicable provisions of the Bankruptcy Code and the orders of the Court.

The filing of the Bankruptcy Petitions constituted an event of default that accelerated Arch’s obligations under the documents governing each of Arch’s 7.00% senior notes due 2019, 9.875% senior notes due 2019, 8.00% senior secured second lien notes due 2019, 7.25% senior notes due 2020, 7.25% senior notes due 2021 (together, the “senior notes”) and senior secured first lien term loan due 2018 (the “Existing Credit Agreement”) (collectively with the senior notes, the “Debt Instruments”), all as further described in Note 14, "Debt and Financing Arrangements" to the Consolidated Financial Statements included in the Form 10-K. Immediately after filing the Bankruptcy Petitions, Arch began notifying all known current or potential creditors of the Debtors of the bankruptcy filings.

Additionally, on the Petition Date, the New York Stock Exchange (the “NYSE”) determined that the Company's stock was no longer suitable for listing pursuant to Section 8.02.01D of the NYSE continued listing standards and trading in the Company’s common stock was suspended on January 11, 2016. We expect that the existing common stock of the Company will be extinguished upon the Company's emergence from Chapter 11 and existing equity holders will not receive consideration in respect of their equity interests.

On the Petition Date, the Debtors filed a number of motions with the Court generally designed to stabilize their operations and facilitate the Debtors’ transition into Chapter 11. Certain of these motions sought authority from the Court for the Debtors to make payments upon, or otherwise honor, certain pre-petition obligations (e.g., obligations related to certain employee wages, salaries and benefits and certain vendors and other providers essential to the Debtors’ businesses). The Court has entered orders approving the relief sought in these motions, in certain cases on an interim basis.

Pursuant to Section 362 of the Bankruptcy Code, the filing of the Bankruptcy Petitions automatically stayed most actions against the Debtors, including actions to collect indebtedness incurred prior to the Petition Date or to exercise control over the Debtors’ property. Subject to certain exceptions under the Bankruptcy Code, the filing of the Debtors’ Chapter 11 Cases also automatically stayed the continuation of most legal proceedings, including the third party litigation matters described under “Legal Proceedings,” or the filing of other actions against or on behalf of the Debtors or their property to recover on, collect or secure a claim arising prior to the Petition Date or to exercise control over property of the Debtors’ bankruptcy estates, unless and until the Court modifies or lifts the automatic stay as to any such claim. Notwithstanding the general application of the automatic stay described above, governmental authorities may determine to continue actions brought under their police and regulatory powers.

As required by the Bankruptcy Code, the U.S. Trustee for the Eastern District of Missouri appointed an official committee of unsecured creditors (the “Creditors’ Committee”) on January 25, 2016. The Creditors’ Committee represents all unsecured creditors of the Debtors and has a right to be heard on all matters that come before the Court.

As a result of extremely challenging current market conditions, the Company believes it will require a significant restructuring of its balance sheet in order to continue as a going concern in the long term. The Company’s ability to continue as a going concern is dependent upon, among other things, improvement in current market conditions, its ability to become profitable and maintain profitability and its ability to successfully implement its Chapter 11 plan strategy. As a result of the Bankruptcy Petitions, the realization of the Debtors’ assets and the satisfaction of liabilities are subject to significant uncertainty. While operating as a debtor-in-possession pursuant to the Bankruptcy Code, the Company may sell or otherwise dispose of or liquidate assets or settle liabilities, subject to the approval of the Court or as otherwise permitted in the ordinary course of business for amounts other than those reflected in the accompanying consolidated financial statements. Further, a Chapter 11 plan is likely to materially change the amounts and classifications of assets and liabilities reported in the Company’s Consolidated Financial Statements.

On August 4, 2015, the Company effected a 1-for-10 reverse stock split of its common stock. Each stockholder's percentage ownership and proportional voting power remain unchanged as a result of the reverse stock split. All applicable share data, per share amounts and related information in the Consolidated Financial Statements and notes thereto have been adjusted retroactively to give effect to the 1-for-10 reverse stock split.

The Company completed the sale of Canyon Fuel Company, LLC (Canyon Fuel) on August 16, 2013. The results of these mining complexes have been segregated from continuing operations and are reflected, net of tax, as discontinued operations in the consolidated statements of operations for 2013. See further discussion in Note 3, "Divestitures".

In response to weak coal markets, the Company has idled or closed mines in the Appalachia region and sold other non-core operating subsidiaries and assets. The results from these operations and gains or losses on the disposal are reflected in income from continuing operations in the consolidated statements of operations. See further discussion in Note 5, "Impairment Charges and Mine Closure Costs".